To Every You I've Loved Before (Light Novel)

In a world where people regularly travel between slightly different parallel universes, Takasaki Koyomi lives with his mother after his parents' recent divorce. Awkward and studious, he has trouble making friends at his new school until his classmate Takigawa Kazune seeks him out. She claims that she’s moved here from another world where she and Takasaki are in love… but could he be the one who’s actually from a parallel world instead?