I'm going to imagine that the main character was living in a 'bad end' type of timeline. So, whatever power sent her back is helping her get out of said bad end. So, I think that if all goes correctly, her senpai and her past self will get together, and she will be returned to the future.
But, she will be together with her senpai. That's probably not what happens, but I thought it'd be the cute way to end things.