Toshishita no Senpai

Nanasawa Nana, 25, has had a long unrequited crush on her senpai, Seto Aya, who she learns has just gotten a divorce. After an "incident" causes her to travel back in time to ten years ago, when Aya was 16, Nana, taking on the alias of Sasaki Mai, decides to make an effort to date Aya in this timeline, soon finding herself competing with her past self.