I am looking for a manga, but I can't remember the name.
A shy high school girl falls in love with a solitary/delinquent boy in the same class as her. They dated for several years but the boy is then killed. A little flying monster appeared and told her that he can send her to the past, just before they meet, and if she agrees to prevent the boy falling in love with her, he will be saved.
So she goes back to the past and tries to do so...