Note: This is a personal review. Therefore, this is much informal and uh... spazzy lol. Please don't mind ^^

Title
Nothing to say lol. It is very suitable for this story. I was thinking it would be better if it was 'The Broken Fated String' but I guess that would spoil the ending of the story xD Better to keep it mysterious instead~

Description & Foreword
On its own, it sounds a bit too literal. I thought a better idea to present the content of your story would be through a question, like 'What if you knew who your soulmate was? Would you conform to fate or would you defy all odds?' And then you can go and bring in Yerin, who is the observer of all these events and also someone who experienced it herself. But this is just my opinion (:

Grammar & Vocabulary
I don't see much problem. Try not to use semicolons inappropriately though. They should only be used if you're linking two related independent clauses (or rather, two parts that can stand on their own as sentences). You also have some awkward sentences like 'Yerin flinched at his question, wanting to look away, but being trapped by his intense glare she couldn't escape from'. I can't really say it's grammatically wrong, but it just sounds awkward.

Writing Style
Generally when it comes to the fluffy and angsty moments, you have it down pat. Humour and emotions are effectively inserted into dialogue when necessary. However, I think your sentences can come off as rather stiff and literal sometimes, mainly because you opt to tell more than show (the good part about it is that it's straightforward and to the point, so readers don't have to pull their hair out trying to understand what you mean). The bad part is that it can break the immersion of the more emotional parts. Maybe it's just my own personal preference, but I think if you focus more on the little actions, gestures and facial expressions of the characters, you can bring out some of the emotions of certain scenes more vividly.
e.g.
During the scene where Myungsoo and Seulyi were sorta arguing, you can reflect the ominous situation by looking through Yerin's eyes. In the eyes of Yerin, who had seen this predicament happen to her parents before, what would this scene look like to her? Maybe Myungsoo's raised tone reminds 
P.S. I also love the repetition you have done, be it for Chanyeol and Yerin's lying habits or for sentence structures. REPETITION FTWWW

Characters
Kudos to you for making these characters feel alive. I'll talk about Yerin, since I love her to bits and pieces. Probably the first time I like a female MC so much lol. Throughout the story, Yerin really seems alive. Although she embodies some typical tropes in writing, the way she is described in this story just makes her so enthralling. Yerin may seem fierce, tough and strong, but that's just merely a facade to mask away her crumbling inner self. While Heejin faces her problems with an optimistic outlook, Yerin faces her problems with a pragmatic POV. She isn't as Hulk-ish mentally as she is physically, but it makes her all the more relatable. Nevertheless, I'm glad she has the strength to move on rather than dwell on the could-have-beens. Despite seeming cynical and nonchalant, she cares a lot about everyone, and it shows. I just wish she would stop acting all tough for once and just let out all the emotions she have been suppressing to... someone. Maybe Chanyeol, or just... anyone. It's heartbreaking that she had to keep all of it in when she was always the one looking out for others and their relationships. As you can tell, I'm very fond of her and having her as the main character of this story is a real delight.

Chanyeol is not so bad himself. He's the exact opposite of Yerin, but I guess opposites attract :) At the same time, they share quite a lot of similarities, which forge some sort of common ground between them. He expresses his concern more openly than Yerin can. One thing I realise about some couples who are polar opposites is that they normally break up after realising they can't get over their differences. Some couples who are very similar can also end up breaking up when they realise that they have the exact same flaws, which end up balling down into a gigantic mess. And a mixture of both of these situations applies to Yerin and Chanyeol. They both assume too much - they assume that just because someone acts ___, they must mean ___. They're too afraid to hurt each other. And because they're not the same, they misunderstand each other. And they end up not communicating. I feel like what has caused their relationship to fail is precisely because they know each other too well, they want to protect each other too much... Too many misunderstandings and too many reasons. I was initially hoping for there to be a miracle, that they would magically reconcile and be together, to the point I can envision infinite (no pun intended) ways they can try to reach some sort of understanding with one another... if only Chanyeol talked to Yerin, or if only Yerin confessed to Chanyeol... but alas I realise life is no fairytale. Nobody is going to make perfect decisions all the time. Chanyeol and Yerin have made many wrong decisions, and as much as I resent them and want them to turn back the clock and somehow communicate to each other their feelings, what's done is done. This is the more realistic picture of how real relationships is like, and while my heart would prefer a happy ending, I think that this is really the perfect ending to leave this story with. 
(I know that, but still, my heart weeps for them... What have you done to me? Lol anyways...)

Moving on to Heejin. The addition of a third party is surprising, but I'm glad she doesn't turn out to be the typical mean girl kind of person (because honestly that's so overdone). However, I feel like the inclusion of her existence is solely just to throw Chanyeol and Yerin's relationship into chaos. Which may or may not be a bad thing. I guess it's more of a good thing, since this is Yerin and Chanyeol's story after all. While she's presented as a perfect image in Yerin's mind, I feel that there's more to her the readers don't know about. It's unfortunate that we can't explore her character much considering how limited the story's boundaries are, but as she is now, I think she suits her role well, and it also emphasises on how HeejinxChanyeol's relationship appears as an infatuation. I feel that Heejin and Chanyeol are together because they want to fill up a certain need that they have, while Yerin and Chanyeol are together because it is... honestly I don't know either, their interactions just feel so effortless. 

Anyway, moving on once again xD

The other side characters are all right. They have great chemistry, although it seems unbelievable that not more drama happens between them (which is what you would normally expect to happen for a large group of friends). The addition of the three new girls seems extra though. It's nice to see Seungcheol (\o/) and I wonder what happened to him after Yerin went overseas lol. I think if they were together, they would have ended up like Heejin and Chanyeol. Which maaay be a good thing realistically speaking, but in my heart, YerinxChanyeol ftw. HAHA.

All in all, the chemistry between your characters is great. The side characters are not as fleshed out as the main ones, but it's fine since the main peeps are the stars anyways.

Plot
I will just name the flaws I've noticed xD First and foremost, I know that this is just a story but the whole idea of Yerin being the only gifted one is... unnerving. And the fact that it's seen as normal. And the fact that its origin is never addressed in the story. It's not a major flaw, but it bothers me since I'm curious about any sort of plotholes lol. Secondly, I don't mind the central conflict being the introduction of a third party (Heejin), but when the whole story revolves around that, it does get a bit weird since relationship problems do not all originate from a third party. I would like it if there was a segment of Chanyeol and Yerin facing problems without having Heejin in the picture yet. And by problems, I mean actual problems and not just the slight annoyance they have towards each other. It would probably highlight the real theme of this story (communication is key) better because the whole love triangle drama can sometimes overshadow the things that really matter (like morals and life lessons).
Nevertheless, I like how you've explored the themes using Chanyeol and Yerin's story ; u ; Good job!

Flow
Not too bad. The events are rather well-paced. I guess you had to rush some parts because some parts of the story can be a little jumpy, but it's not too distracting. 

Overall Enjoyment (10000/10)
No lies lol. I honestly didn't expect to enjoy it so much but I guess I'm reading this at a good timing, because Yerin and Chanyeol really remind me of my own life story HAHAHA. And throughout the whole story I was like 'O-M-G I FEEL THIS' or 'SDVHVM WHY YOU DO THIS TO ME T_T'. I actually cried which is pretty surprising... I don't usually cry unless I'm really invested in the story lol. Not gonna talk about my whole life story since it would be a loooong one but because of this one person, I really started believing in 'fate' (even though I thought the whole idea of fate is ridiculous beforehand xD), but at the same time, I feel that we can never progress into an actual relationship... because of so many things combined. I think he'll still hold a special place in my heart though, just like how Chanyeol is to Yerin. And I've heard of how relationships break like Yerin, so yeah... Obviously I can't see people's fated strings and stuff but I can really relate to this story. You're a super good writer T_T I'll probably check out some of your other stories if I do eventually have the time because two of your stories so far have been giving me the feeeeelz. Keep it up! And if you ask me, I think you can actually publish this story as a book if you just expand on some parts of the story.