Salutations!
Okay so, I read through some of your novel and I've accrued some feedback for you as per your request!
One of the first things I wanted to know was, is this story a diary or is it in First Person POV? Sometimes the novel is written like it wants to be a diary entry and sometimes it's written like it wants to follow First Person. There IS a difference between the two and sometimes going back and forth can break flow when you write. Fear not, I have examples from your text to illustrate what I mean!
Diary Entry: As I laid down on my bed, I realized that I couldn't remember anything that happened after the time I had breakfast. I couldn't understand why at the time, but after I did some research, I came to the conclusion that only one thing could be the cause of my memory loss.
First Person POV: Everything started to fade and I felt dizzy. My eyes started tearing up and my heart was beating so fast that it was about to pop out of my chest. My body temperature started rising but my sweat felt cold. I didn't want anyone to notice yet at the same time, I wanted someone to save me.
I chose two emotional moments to show you how they differ even though they seem similar enough in format. In the Diary Entry, we get a run down in the character's voice about a concern. It's written like you would see it in a diary so, kudos to you because this does seem like something a teenager might write in his book. He conveys a past concern as if he's lived it, rather than if he's living it in the moment so, very diary like.
In the First Person POV scene things are different. We're not hearing about the event as if he's already lived it, we're hearing about the event as if he's in the act of living it in that moment. This is what I mean when I say that we've run into a POV issue. Sometimes it's a diary, and sometimes it's not. If he were writing a diary entry about what'd happened, instead of using words like "Started" or "was beating" we'd see past tense words used. I rewrote this second section to illustrate how that would look so you can see it from the Diary Entry POV:
Edited: I'd felt dizzy. My eyes teared up and my heart beat so fast I thought it would pop out of my chest. My body temperature rose but my sweat was cold. I didn't want to be noticed yet at the same time, I wanted someone to save me.
See how we use past-tense words to make the illustration of the scene? And how they flow more like someone was writing them after the fact, not during? That's following along more with the diary format. So, essentially I'd just mind your POV and determine if you want it to be a diary entry or in First. Or if you wanted to have one section be a diary entry, and another section be in First, I'd just label the diary entries as such so your audience knows when the switch happens, that way it won't break immersion or flow. When you want to write in First Person, you also have to bear in mind that you need to avoid the use of Passive Voice, which is what you use when you write diary entries. What do I mean by that? Passive Voice is the writing method you use when you TELL something rather than SHOW it. I'm taking some examples from your text to show you what I mean.
Passive: At that moment, I was so happy that I could see him smile like that at me. I began to notice how cute he was again. Some time passed and we were basically done with the first part of the project. It was late, so we decided to take a break before I'd leave.
Active: His smile lit up my heart and I noticed how cute he was again. Some time passed and we finished the first part of our project; we took a break before I had to leave.
If you're going to use the First Person POV you'll want to keep your audience in the action with you, rather than treating the action like a diary entry, that way the scene will evoke emotion from your readers, rather than just being a list of events in order. That's the one thing about Passive Voice and diary entries, you run the risk of not connecting emotions to your readers, so it's something to bear in mind depending on how you want to write your story. The diary entry format is a difficult one to balance but, with editing, I know you could get there.
Now, I'm sure you noticed that in two of those earlier paragraphs I transcribed, there were some words in bold. This I've done to illustrate another issue I ran into: Overuse of words within the same paragraph. So, often you'll repeat words or phrases in the same paragraph. This can break narrative flow and distract your reader. If I start seeing the same word or phrase crop up within sentences of each other (without there being another narrative convention like purposeful repetition or something of the like), I get derailed by it and have to refocus in order to keep reading, however, this also turns my attention to the problem of repetition, so if it keeps happening, I'd stop reading.
We've also run into a 'Character Soup' issue. This happens when too many characters are introduced at once and aren't defined enough for us as readers, to be able to tell them apart when they're on scene. In Chapter 3 we're introduced to Hayato, Nao, and Tarou. It was also at this point when the narrative shifted focus from being in Yoru's voice to being in Hiro's, so I know that he had his own set of friends he was trying to introduce Yoru to. However, they were all introduced in such a rush I actually had no context for who was who and why they'd be important later. This might be something you could spend a little more time on to flesh out the relationships and make these other characters feel like they're part of the story too.
I will also say, that, when reading both Hiro and Yoru's sections, they sound like they're the same person. Now, again, this is the tricky part of First Person POV. How to get a different voice in a separate perspective. I didn't really get a sense of Hiro's identity as I did with Yoru in that, Yoru introduced us to his depression and self-deprecating thoughts. Hiro spent a lot of time on his relationship with Yoru to the exclusion of his own traits. This could be a stumbling block, because you don't want the relationship to be Hiro's only identity. You want to make sure he's got his own voice and that his defining traits go beyond just being Yoru's potential boyfriend. Almost everything in Hiro's section, was about Yoru, rather than Hiro's day-to-day. Just something to keep in mind as you edit, don't lose Hiro's identity, you want him to be his own person.
Like with most editing issues, this is all stuff endemic to first drafts, so don't worry it's not as though you're the only one to run into these things. These are just some of the issues you need to look out for when you go through and edit, so you don't have to stress about missing something. Don't worry, you'll get all this stuff handled in due course! And to help you along, I brought tools! (Hah, when do I not have tools?) First off and my personal favorite: Editminion! This is a tool that will help you find weak words, Adverbs (in the sections where you want the action in the moment, you'll want to get rid of adverbs), or repeated phrases. Here are some quick tips about writing in First Person. Here's an article about writing a story in the Diary format. It's a short article but the site also has other ones that you could read for other tips, if you find any that might be handy for you.
I think you've got some serious potential with your story and here's why: You have 3 POV characters, Hiro, Yoru, and the Diary. You get to show your audience how each of these guys experiences this relationship AND you get to see Yoru's writings on the subject (which, depending on how he feels, could differ from what happened if his feelings change. This is a very interesting idea that you can flesh out here). Once you've started your editing process and you start shining up the small details, I think this collective of perspectives could turn out pretty strong.
I wish you luck and I hope this feedback helps you as you go through your editing phase.
Also, fair is fair, you're more than welcome to check mine out and offer your insights as well!