Because he was hurt and wanted to forget. Not to mention it's been what, 9 years? Thats a longer time than people realize.
I also wonder if he didn't actually get physically hurt since it mentioned him slipping before and later it mentioned he was having seizures.
i agree with the other comment. from dohyun's pov of the flashbacks its clear he only managed to remember the good memories from his past (well humans tend to forget hurtful memories) and sungho left him and said something to him ("its nasty") when he thought the relationship he had with sungho was going well (cuz dohyun did love him). this hurt dohyun (i dont quite remember but dohyun said something abt sungho being like everyone else, was never there for him and would eventually leave him) so thats why it became a bitter memory for him and made him unable to remember
Oh, i think i get it now. Thank you so much ヾ(❀╹◡╹)ﾉ~
Maybe I'm too stupid to understand but why did Dohyun forget about their relationship? I mean, if somebody really love someone I doubt he can just forget that person.