Recap

i dont remember everything either, but i think what caused them to fight is that mc left the file ab ml on his computer open (he intended to delete it but i think something came up) and ml saw it and ended up whether anything they had was even real

In the beginning of the story the whole reason Dohu follows around Juheon is because he wants to study him and use him as inspiration for the character he is trying to write.
However do to a misunderstanding, Juheon thinks that he's being followed because Dohu was interested in him romantically.
They eventually end up together and long story short, Juheon opens up about how he's used to being used by people and is so glad that Dohu loved him for him.
At this point Dohu is no longer using him as character inspo but he also hasn't confessed to what his intentions were from the beginning because he's worried that doing so will just make things worse, and that it will be better for Juheon if he doesn't find out.
Eventually Juheon does find out on his own because he finds the tab on Dohus computer where he was taking notes on him, and he questions if at any point during their relationship Dohu really loved him or if he was just playing the long game to get more info out of him.
Dohu doesn't really give a proper apology which causes Juheon to leave him and Dohu doesn't really put up a fight to make things right until the very end where he then tries to run to Juheons apartment, but by then Juheon has already up and moved away.

P.S. Juheon actually seemed to just maybe reconsider breaking up and talking things out more, but his aunt threatens him with what we can only assume is a photo of him and Dohu being romantic, which causes him to go abroad like how the grandfather wanted.