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.