i heard it from a friend but he himself wasnt sure were he got it from.
there are only few details i got:
first theres a male lead who chases after a girl though she doesnt like him or is just neutral to him. after some time the protagonist gets a girlfriend but not the girl he chased before. his girlfriend later on dies in an accident or disease or disappears i dont know but i think she died.
i think after that romance developes for the hero and the girl he had a crush on.

its very vague and its likely that some details dont fit
does any manga come to your mind? it probably was an anime

Apart from a few details which aren't the same, it sounds almost exactly like "Kimi ga Nozomu Eien," which is just an anime. As far as manga goes, nothing really comes to mind.