Help me look for this bl novel

I'm not sure if It's a transmigration to a book or a rebirth. But it's chinese bl I think. And at the end there's like a side chapters where it shows what would happen if the characters didn't meet. Fudgee can't remember much but it's a good onee really.

What I really remember is the part that one of the character (MC or ML) celebrated his birthday with others and he feels kinda sad and empty (maybe because he's suppose to be with the other one yet he's celebrating it in the world without him)

Im not sure if it's a side story or it's really the main story. Ackkk. If you know it pleaseeee tell me, or anything similar bl plot. Thankiees