Title: 

Initially, I didn't really fancy the title since I had been seeing too much titles with the word 'last' and your story's title was no exception. So honestly, if I see this in the latest updated fanfics, I wouldn't click on it as it wasn't attracting my attention with the word 'last' in it. However, I did question myself if there was anything that was going to happen if the last petal drops and I decided to check it out.

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Plot: 

I really love your plot-line where Yixing reads the dairy of Sera. I love it because I just enjoy reading dairy entries and this one-shot reminded me of some books like 'The Diary of A Wimpy Kid' and 'My Dear Dumb Diary' but this story's atmosphere was more about the romance of both Yixing and Sera. I did find the encounter quite cliche, to be honest, because in shows and books, couples normally meet when they asked directions? So, I was quite upset when I saw that since it was over-used and I did expect a more creative and unique first meeting for couples.

As for the day where Sera became Yixing's girlfriend, I found it very sweet and touching when he confessed on Valentine's day, like how cute is that? I found this part of the story cute and nice. However, I do have to point out some stuffs where I found un-realistic. Let's have a look at them. In Sera's diary entry, one thing I found unbelievable was that Sera calls Yixing by Lay. That is sort of a stage-name for him. Since Sera was so closed to Yixing, she should be calling him by his real name. So, I found this part very unrealistic. 

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Language:

 I did found some mistakes while reading your one-shot so let's have a look at them.

Original: "After we finished all those movies, I bragged him to a flower shop and I bought a flower."

There is a difference between 'bragged' and 'dragged' because bragged meant that see was showing-off something to Lay and that was definitely not what you meant. So, it should be 'dragged him'  instead. Please take note that you have to use the correct word because it did make the reader lost when you used a different word. Other than this careless mistake, I still find the sentence awkwardly-structure. What does 'after we finished all those movies' mean? It sounds awkward, weird and I couldn't get any sense from that part of the sentence. 

Next, you started repeated the obvious facts again. 'I dragged him to a flower shop and I bought a flower.' Can't you see that you are trying to make the story slower and draggier? I didn't like the pace of the story after you started adding things that you have already mentioned. It really annoyed me so please stop doing that.

Edited Version: "After we finished the movies,  I dragged him to a flower shop and bought a flower."

Talking in terms of description, I did found this sentence vague and unclear. I would appreciate the story if you did include more details like where was the flower shop located? Near Yixing's house. She bought a flower? What type of flower? I did think a lot when I read this sentence, it was again unrealistic because normally, people usually buy a bouquet of flowers instead of just a stalk of flower. So, I will recommend writing that she purchased(a better word instead of buy; buy is too common and easy for your standard) a bouquet of fresh lilies that gave off a fragrant smell.

For tenses, you did make some mistakes too. So, you should have a look at them.

Original: "He reminisced back the old day, smiling widely at the sweet and remarkable memories."

It should be in plural form for 'day' since they did spend a considerable amount of time together, thus it should be in plural form. Do take note of your tenses because they mean a lot in English writing.

Edited Version: "He reminisced back the old days; smiling widely at the sweet memories they had."

I did find your writing awkward since you sounded a little like Sera in the story. She did have problems expressing herself and you too. You seemed to have a hard time describing parts of the story and when I started reading them; I felt so awkward. Here's an example that you wrote.

Original: "I hope I can tell him that I really love him and I want him by my side forever but I know that I’m not that type of person. But one thing for sure, I’ll do anything as long as he’s happy."

Read it several times and you're understand what I mean. What do you mean by saying that 'I know that I'm not that type of person', the sentence structure did not even flow and it had bits and parts of broken English in there. You can see the repeating of 'but' in there and like what I will usually say, I don't like that.

Edited Version: "I do hope that I can tell him one day that I really love him and wish to stay by his side forever. However, I did had problems expressing my heartfelt feelings since I didn't really have the talent for speech. I am certain of a fact though; I will do anything as long as Yixing is happy."

Isn't it better now? I do find these few sentences flowing in a better manner. 

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Characterization: 

For this area, I am hesistating whether if it was great or was it weak as it has both pros and cons in the characterization. Initially, I did disagree in the characters' personalities that you have given each of the character because it was indeed very popularly-used in the fanfic world