And the story has them getting back together in the end.

Them having to go through an ordeal such as a love triangle to get back together. Maybe one or both of them are in other relationships, but then fall back in love with each other.

I think maybe Ao Haru Ride might fit the bill? It's been a while since I read it - but I'm pretty sure they were together - or at least liked each other/ were more than just friends and then had a falling out before the manga starts.

"The man who smiles when things go wrong has thought of someone to blame it on"
- Robert Bloch

Koi Shitagari no Blue follows this template except for the first part, where the two MCs aren't in a relationship prior to the beginning of the story.