A girl is sad about her best friend’s death, a guy, and she reads his diary. In it, he says all the things he wished he could do but didn’t due to his terminal illness. She goes through this door in a house on the floor, I think, and she’s back in the past.
She sees her friend and tells her past self to do these things. She’s back in the present with him dead, but she has new memories and his diary says he fell in love with her and he’s thankful for her help. I remember they climbed a mountain he couldn’t at the field trip before.
So after looking at a few thousand profiles of something that might be what I’m looking for, I think (emphasis on think) the blurb said the girl saved him from being bullied but he didn’t need help. This might be part of a different manga but hopefully it isn’t. I also recall he meeting him in the past on the bus? My memory is so foggy but this manga is lowkey vivid. Possible a one-shot.