[Warning: I know you said you don't mind a harsh review but still, nothing here is meant to offend! I just tried my best to offer my most honest opinions, and so... this is it. Please don't mind the harshness!] 

FIRST IMPRESSIONS (8/15)
Title: 1/5
`Attractiveness: 0/2
Okay, the thing is, the title is very straightforward. There's no double meaning to it, so it doesn't keep the reader guessing. And not to mention, the premise that surrounds an ex-boyfriend (breakup) is very common. The title itself is also vague; it indicates that a breakup happened, but it doesn't hint about the details or provides additional information that can hook the readers in. Sad to say, but when I saw the title, I really wasn't hooked in at all. 
`Relevancy: 1/2
The breakup is the main focus, so yeah, the title is indeed relevant. The only problem I have is that it's just a very shallow title which doesn't fully bring out the complexities of the feelings involved in the breakup. So if you think about it, the title is relevant, but relevant on the surface level.
`Originality: 0/1
It's not original, unfortunately.

Description & Foreword: 3/5
`Attractiveness: 1/3
Nice contrast between the initial romantic moments and the later harsh breakup. I have mixed feelings about the inclusion of the last four sentences, because you're pretty much giving away one plot twist already, and it seems like that (along with Jongin's reason for the breakup) are your major plot twists. I'd advise you just end at the first paragraph (and maybe add a concluding question, e.g. 'Why did Jongin break up with Kyungsoo? Could they ever repair the broken remnants of their relationship, or would they find solace in someone else instead? (This is to tie in with the love rectangle of the story). But other than that, the description does evoke the readers' curiosity as they wonder about the reason behind Jongin's reason behind initiating the break up with Kyungsoo, even though it does present a very cliche premise by itself. I really dislike the character descriptions though. They provide very shallow bits of information about the characters and can create a false impression of the characters. The preview is fine, since it kinda explores the overwhelming grief that Kyungsoo experienced when Jongin broke up with him (which is pretty much the central part of the story). But I'll admit, the description generally looks like it's going in a very cliche direction (partially due to the premise itself), so the description didn't really catch my eye.
`Relevancy: 2/2
Pretty relevant, and it touches on the emotions that Kyungsoo faced as well, so kudos to that at least.

Format (Graphics & Layout): 4/5
`Graphics
I like how the title separates Jongin and Kyungsoo, and how they're looking at opposite directions wistfully (as if at each other). It really symbolises that the bond between them had broken. The poster is simplistic but it brings the point across. Just a small note: There's really no point with the whole Chansoo vs Kaisoo concept when it's clear the whole story revolves around Kaisoo. The title and the graphics all scream 'Kaisoo' at me. If you're planning to divert into a different direction/want to keep things mysterious, you may consider switching the title and graphics to make it seem less Kaisoo-centric.
`Layout & Formatting
I do like the way the chapter titles are stylised, but throughout the story, the main body has changed different fonts and sizes. Perhaps it may be better if you just use a uniform font and size, since the inconsistencies can be quite distracting. Also, try to avoid using bold. It's just not a professional way of formatting. Use italics instead if you want to emphasise something.

WRITING STYLE (5/25)
Grammar & Vocabulary: 3/10
`Grammar: 2/7
``Terminology
[T] - Tenses. [ss] - Sentence structure. [SVA] - Subject-verb agreement. [pu] - Punctuation. [P] - Preposition. [ww] - Wrong word.
Know your problems very well so that you can learn how to correct them.

``General overview
Kudos to you for using semicolons correctly! Your sentences are generally complete even if there are awkward structures. Your tenses are very inconsistent. Your main tense is past tense, but you would switch to present a lot. There are also SVA errors. Occasionally, you would also string complete sentences together using commas, when they should be separated by full stops. You also miss out the closing punctuation for dialogue. Be careful with your usage of apostrophes as well. You also have other miscalleneous errors here and there (typos too), but your main problems are your tenses, prepositions and sentence structures. Generally, I can see great improvement from your earlier chapters so good job.

``Examples
C1: "Even if he knew he hadn't gone to school in [P] a few days already, and if he continued to skip his classes, he'd be expelled." - There's no logical flow when you start the sentence with 'even if'. 'Even if' indicates that scenario A will still happen no matter what (even if scenario B happens). Over here, scenario B is him knowing he hadn't gone to school, and scenario A is him being expelled. How is it that him being expelled will happen no matter what? It won't happen if he attended school, so that means there was still a chance for him to redeem the situation. So 'even if' is unnecessary. 'in a few days' means that he wouldn't go to school in a few days time, which I'm pretty sure isn't what you meant. "He knew he hadn't gone to school for a few days already, and that if he continued to skip his classes, he'd be expelled."
C2: 'that's why it [SVA] has [T] always been haunting him and hurting his heart so much' - The previous sentence indicates plural form, so 'it has' should be corrected to 'they had'.
C2: "It's okay, [pu] let's meet at [P] lunch break, okay?" - You should a full stop when you are separating two independent sentences. Also, preposition is a little off here. You can say to meet at a certain place, but when it comes to timing, you can only meet during a certain time.
C10: "The silence filled out [P] the entire living, with Jongin who was sitting in [P] the couch." - 'filled up' is a fixed combination used to describe filling up a space. 'with' is not necessary here since it kinda gives off an 'along with' meaning. Jongin can also only 'sit on' a couch, and not 'sit in' (there's no space inside the couch for him to 'sit in'). "The silence filled up the entire living room. Jongin was sitting on the couch."
C10: "He just couldn't stand to see Kyungsoo cry and as much as he wishes [T] he'd die." - 'as much as' is kinda unrelated here since there's no follow-up explanation on that point.
C11: "Seeing how much anger Chanyeol has [ss], that told [ww] him that his nice and gentleman image was completely erased from people's minds." - The phrasing of 'how much anger Chanyeol has' isn't technically wrong, but a little awkward, so I tried to rephrase it. 'anger' cannot 'tell' a person something directly, so you have to use another way to convey what you intended to express in a correct way. Also, it's better to say 'nice gentleman image' since gentleman isn't an adjective like nice. Note how the two parts of your sentence do not have an appropriate link. Even if Chanyeol was angry, that doesn't mean other people were angry as well. Take note of the logical flow of your sentences. "Seeing how intense Chanyeol's anger was, he knew that his nice gentleman image was completely erased from people's minds."

`Vocabulary: 1/3
You have a very simplistic way of describing things. I've seen 'sad', 'angry', 'pain' etc countless of times throughout the course of your story. The bigger problem is that they are extremely vague words that cannot fully express the emotions you want to convey. Hence, your vocabulary needs a bit more work. I've seen improvement in your recent chapters though, so keep it up! Your sentence structures are also mostly dynamic. 

Writing Style: 2/15
`Description: 1/5
Your writing style is quite inconsistent. Chapter 1 started with lots of dialogue and little narration, while your latest chapter has a lot more narration and description. It makes sense since you're still a growing writer, but the lack of consistency in the writing style can be jarring. It would be great if you can take some time to rewrite your earlier chapters again. Due to the inconsistency, I'll judge based on your most recent chapters which I believe are a better representation of your current skills. 
You have a more literal way of narrating, in the sense that you would merely describe the general picture of the scene. However, there's a severe lack of details, and you don't have anything else in your writing that can help you convey a certain message to your readers, like literacy devices (though they're used occasionally), language use, 5 senses etc. This is generally not a big problem since your story is more emotion-centric, but even the exploration of the characters' emotions is insufficient (more to be said in the Writing Style/Emotions section).
However, the bigger problem is that we don't see much of the flashbacks. Scenes are mentioned and brought about in a shallow manner (e.g. Jongin and Kyungsoo's first 'actual' meeting, the development of their relationship). Explore them more. Rather than simply repeat 'oh, Kyungsoo was sad' for the 5000th time, it's better if you allow him to recall about their happier moments (concrete ones) and wonder about the what-ifs. That would be more impactful that using a single word to state how he was feeling. 
Like I mentioned in the Vocabulary section, your words are extremely vague. 'sad', 'pain' are words that are often used, but you never go further to describe the intensity of these emotions, or use other details like actions, expressions etc to express them. Even the change of how Kyungsoo views his surroundings and people can show these emotions. But your story just doesn't show that, and that's rather depressing.

`Emotions: 1/5
Your emotive dialogue is pretty fitting for the most part, so that's a good thing.
Like what I mentioned in the Characters section, you barely describe the characters' emotions very much. 'Sadness', 'happiness' and 'anger' just don't cut it. It's easy for someone to say that they're sad, but the word itself is so overused that it has become bland. You need inventive ways to present this 'sadness'. Only then can your readers feel for your characters. Furthermore, you don't fully display the complexities of your characters very well, and that's a major problem that many writers have in AFF. You're only stuck with the basic emotions of 'sadness' and 'happiness', but you don't explore the depth of these feelings. Sadness can mean various kinds of sadness. They can be followed with disappointment, anger and resentment. But you don't really touch on them much, and again, that's a real con.
Additionally, there's way too much draggy 'angst' going on in this story. We see Kyungsoo still sad after 20 chapters, and it gets... monotonous. There are little flashbacks that can express enough happiness to balance out the negativity, and instead, you're just stuck with sadness after sadness for 20 whole chapters. There are some good writers who can make the prolonged angst work, but you still lack the skill/ability to make this sadness sound genuine. It mostly feels like I, as a reader, am forced to take in the fact that Kyungsoo is sad everytime he appears on the screen, and I end up feeling fed up. This is why I end up being frustrated over this story rather than actually feeling pity for the characters. The angst is way too much, and it's not done tactfully enough.

`Memorability: 0/5
All I can say is... generic. I wish I can latch onto something that can make me remember this story, but I'm sorry to say this... I just can't (unless my frustration counts as something). Your narration itself is generic, but that's pretty common in authors of AFF. The bigger problem is your dialogue. It's stiff and awkward, and it only sorta shines when it comes to conveying more negative emotions (like the scene with Chanyeol and Kai is good). Even then, it's not exactly memorable. I guess this is probably because there's a severe lack of humour, even when it comes to the fluffier scenes, and it's... kinda weird. Sure, this isn't the happiest story ever, but humour is a very essential part in our lives and it livens up the characters. That's why ultimately, I can't give this section marks.

DIGGING DEEPER (10/55)

Characters: 3/25
`Believability: 2/10
``Do Kyungsoo
Although Kyungsoo is very frustrating to read, I think that he's actually pretty believable on a surface level. I can imagine many people crying for days after a breakup with their boyfriend, and the torment he underwent everyday seeing his ex's face must be immense. :x His character stays constant throughout, frustrating but nonetheless passive. I do like how concerned he is for his friends though. I may sympathise him more if the story tries to talk about what he actually did to vent his emotions (other than crying), and if he even tried to listen to his friends' advice. It seems that he was merely dismissing them off, not listening to them at all. He doesn't even try to change. Which makes no sense considering his passive personality.
I thought that him being so nice to Soojung, who was nothing but a prick to her own brother, is kinda unbelievable, even when you dig into his past with her. None of it is even his fault, so why exactly does he deserve such treatment from Soojung??? I just wonder if there's ever a limit to Kyungsoo's patience. No one can ever hold everything inside oneself, and more importantly, no one can ever be as 'saint-like' as Kyungsoo to do that.  It's so clear how much pain he was in when Jongin broke up with him. Yet, for someone as reliant on Jongin as him to be okay with Jongin dating his sister, who had been treating like poop and calling him the 'adopted son', and then being okay with being 'friends' with his ex for the sake of this same person? You have to admit that's way too much. 
This may make sense if there is a reason behind Kyungsoo's strangely placid personality, but his backstory doesn't indicate any signs of abuse that may hint to the shaping of his personality. Besides that, I'm concerned by how the story makes Kyungsoo's personality seem okay. I know he loves his sister and Jongin a lot, but loving someone doesn't mean you would take from them. Both Jongin and Soojung have been so cruel to him, yet he simply just nods and takes it all, like he's some punching bag to be punched around. From the way his character is developing, it seems like he has terribly low self-esteem of himself and needs someone like Jongin to validate his reason of existence. However, the story never explores more intricate issues like this and merely stops at the surface level of Kyungsoo's personality, which is passive, forgiving and loving. Hence, this feels like... a Kaisoo story only. What do I mean by this? It feels like this story exists to ship KaiSoo together without developing the characters themselves as humans, or tackling real social issues. Which is fine for Kaisoo shippers. But if I look at your story independently, without regard of the shippings involved, I can't feel for the characters or plot at all. T_T

``Kim Jongin
He's more unbelievable than Kyungsoo, and I think that's largely because we don't see much of his perspective. You share a similar problem with the previous request, but over here, I simply have zero clue as to why Jongin broke up with Kyungsoo before the 'His Reasons' chapter. That just shows poor foreshadowing. It's not just that. He broke up with Kyungsoo, then proceeded to kill himself because he was that devastated and so hung up over him... yet still decided to date his SISTER knowingly. After that, he even asked Kyungsoo to be 'friends'. Seriously? Why exactly???? It probably has something to do with his mom. Perhaps, his mom forced him to break up with Kyungsoo and decided to make him date Soojung instead (?). I can imagine him doing the first option, but definitely not the second one. Why would he torment Kyungsoo by dating his own sister? There's no way in hell Jongin would do that. I imagine he'd settle for some random girl if he is forced to do so ultimately. It's just... For someone who seemed to have loved Kyungsoo so much, I can't comprehend his actions at all. Just please make me understand if you can.
Jongin also got jealous really easily. I find it funny how he misunderstood the scene in C2 when it's obvious that Kyungsoo was actually hobbling. Why is his priority 'oh my god, I can't believe that jerk Chanyeol is helping my ex out' rather than 'oh my god, why exactly is Kyungsoo hurt'? 
Also, unlike Kyungsoo, it feels like Jongin didn't care about the people around him at all. He kept worrying people in C8, brought Soojung into his own love problem with Kyungsoo even when he didn't like her, and... killed himself. Speaking about that, I wish stories would stop throwing the 'committing suicide' topic so lightly like that. There have been people who committed suicide because of their lovers, yes, but again, the portrayal is key. The story merely uses that topic to enhance the 'dramatic'/'tragic' effect, instead of delving into why Jongin did that or the negative impacts of such an action. And so, this story feels a lot like melodrama. Lots and lots of melodrama. 
P.S. Why did he want to make people think he was an ? Actually, it doesn't seem like he was planning to do that because only Chanyeol and Kyungsoo treated him like an . Everyone else still worshipped him. 
P.S.S. The way how Jongin followed Kyungsoo is just... wrong. Love in first sight does happen, but it's so rare and... who actually follows some random person they barely know? No matter how you like them, that is so wrong and unhealthy, and is in no way 'love'. Jongin's stalking is just romanticised to fit the desires of shippers.

``Park Chanyeol
Chanyeol is really biased, but I guess that's human nature. He's pretty all right for the most part. Judging from his personality, I'm surprised he never snapped at Kyungsoo, because I think most people would get fed up with Kyungsoo's melodrama (me included... I'm sorry T_T). His patience astounds me. Then again, he probably did know Kyungsoo for a long time, and him being his crush helps.

``Do Soojung
Soojung is horrible, but I kinda understand her. She probably has reservations against Kyungsoo because he's her adopted brother, and also because he's homoual. But it also goes to show that she never truly loved Kyungsoo as a brother. At least, no real sister would jump on the bandwagon like the rest of society and start calling her brother names just because of his ual orientation. 

``Others
I find it weird how everyone seems to know one another lol. Magic of being in the same universe. Other than that, they were quite all right.

`Portrayal: 1/10
``Do Kyungsoo
Kyungsoo is portrayed as a very demure guy who cares for his loved ones and doesn't like to bother him with his own troubles. Many people in the story reiterate that fact, but from a reader's perspective, I really feel that he's just a coward. He kept everything to himself, but at the same time, he never did anything to overcome his troubles. He only kept crying, making it difficult for both himself and the other people watching him, because it just feels like so much melodrama. It may be fine if there's only Kyungsoo in the whole cast list, but combine Kyungsoo with other characters who are equally melodramatic and this story becomes a pain to read T_T His portrayal is very typical for a damsel-in-distress role. I wish he has stronger views... stronger opinions... dynamic emotions. Yet, he's very placid throughout.

``Kim Jongin
He loves Kyungsoo a lot. However, there are many things we don't get about him. His stalking, his intent to become an and everything... They're all veering his character into a bad light. Some characters are intended to be the bad guys in stories, which is perfectly fine. However, Jongin is clearly not meant to be portrayed that way, and in fact, even if he is, there are a lot of confusing things about him. He certainly seems more daring and outgoing than Kyungsoo, but I wish we know him more as a person than just 'the main character's ex-boyfriend'.  

``Do Soojung
She paints a stereotypical picture of the public, which I thought is a plus, but her 180 degree change of attitude towards Kyungsoo is baffling. That makes her seem more like a stereotypical antagonist than a representation of an involved party in a real social issue.

``Others
Frankly speaking, they're all quite unmemorable. I blame the fact that they fit into stereotypical roles too well.

``Kaisoo's relationship
Their interaction, from the flashback, is very sweet, but kinda bland. They're super romantic with each other but their interactions don't seem genuine. Stiff and awkward. I feel like they don't have much chemistry. This also applies for Chansoo. Both Kyungsoo and Jongin don't even trust each other enough to address their problems in front of each other. While they may be worried for the other party, it's also evident that there's a lack of communication between them which is a clear sign of unhealthy relationships, not to mention they're both incredibly reliant on the other. I wish there's further exploration on how they first fell in love with each other (the fluffier moments) to further substantiate their relationship. But no stalking, please. That already isn't a healthy start.

`Development: 0/5
``Do Kyungsoo
There are times when he stood up to confront Jongin, but there's barely any actual development of him as a character.

``Others
No development at all. Jongin killed himself, yet he didn't seem greatly impacted by that incident. Neither was Sehun. Soojung regressed as a character when she found out Kyungsoo was homoual, so that's a nice touch. But other than that, the characters are mostly static.

``Relationships
I would like to see development on these the most, since they play a major role in your story. How does KaiSoo's interactions change? What about Soojung to Jongin, or Kyungsoo to Chanyeol? They have potential to grow, but for now I don't see how they've developed in the story.

Plot: 5/25
`Theme: 2/10
I suppose it has some elements of 'ostracising homoual people is wrong', but the focus seems to be more on the romance, which is a pretty shallow concept. Nevertheless, it can be done well if execution makes it work. However, the actual romance between the characters suffer other problems like lack of development, lack of authenticity etc, hence I don't feel much for the romance. Additionally, I'm clueless as to what the breakup's impact on the characters and their relationship are... Is it that unconditional love is important? Or is it that we should never give up on love? The direction isn't clear, so it makes the whole story feel bland and static. There's definitely angst and drama; however, there's way too much of it to the point that everything feels like a melodrama.

`Portrayal: 1/10
There is very poor foreshadowing of the reasons behind Jongin's breakup in my opinion, but feel free to let me know if I've missed out some things (since I'll admit I'm terrible at picking up hints). While there are several conflicts, they're just... super cliche. Breakups have been done a couple of times, yet you've not brought anything new to the table regarding them. Love rectangles have been done a couple of times, yet you've not brought any gripping emotions that may change my views towards them. Flashbacks are not introduced well, nor are there any outstanding elements that can make this story a remarkable one. The plot twists and motivations of characters are all very typical, bland, so this feels like a Kdrama. I think the mentioning of homouality is a nice touch, yet you never really explored on that much, so... the story remains like this. 

`Realism: 2/5
The chance that everyone knows one another is highly little. Also, Jongin dating his ex's sister? That sounds planned, but also unlikely... Like I can't imagine why Jongin would do that, unless he was instructed by his mother or something. There are some other moments that appear almost kdrama-scripted, but other than that, the plot seems okay.

Flow: 2/5
The angst is way too draggy, yet there's insufficient fluff, new plot twists and substantial development that can make it a worthwhile ride. 

FINALLY (0/5)
Overall Enjoyment: 0/5
Honestly speaking, I really can't bring myself to enjoy this story. In fact, I felt very frustrated while I was reading, because even though I want to understand the characters, I just end up feeling like punching them in the faces. I think I may've quitted halfway if it wasn't for the fact that I want to read the whole story to have a better understanding of why Jongin did what he did.

OVERALL ANALYSIS
Strongest aspect/s: First Impression
Weakest aspect/s: Characters, Writing Style, Overall Enjoyment
Technicality Level: 1/5 - There are many flaws in the writing style and plot.
Feels Level: 0/5 - Chemistry and writing style. They affect this a lot.
Longevity Level: 1/5 - I did feel like going back just to check out if there's anything I missed, but thinking about the heavy angst makes me kinda hesitate for a moment.
Memorability Level: 1/5 - I can remember it... for the many frustrations this story has. But many stories have done that for me so far, and this isn't really an exception.

TOTAL SCORE: 23/100
Comments: Overall, the story is ringing out to me in a very cliche and frustrating way so that's something you may want to work on. Still, I just want to commend you on how much you've improved from C1 to the current C17. There's a dramatic change in terms of your writing style, and it's a positive change. I may have given you higher marks if your writing style is more consistent throughout the story. I do hope this review has been insightful, and you're at least more aware of what you need to improve on. Happy writing!