Looking for a manga where the mc is on his way to a wedding when he dies (don't remember how). He is turned back to when he was still jn HS, but not as his younger self as you would think. But as a (very small) spirit, and he tries to tell his younger self about one classmate that he shouldn't get close to.

Because, acording to him, he was the one that messed his life up. And it was also to the classmates marriage he was on his way to, more out of spite than anything else.

Found it. https://www.mangaupdates.com/series/7bvbjlb/zero-kyori