I need Recc's, please :)

There is a book that I would love to read, I just haven't discovered it yet.  It might not even exist but I've been looking for a long time and decided to ask here...


I'm wondering if anyone has read anything that sounds even remotely similar to what I'm looking for.  I don't have a specific book in mind and I don't know if there even IS a book such as this but if there is, I really want to read it.


It's probably got limited appeal to most people but I'm hopeful it's out there somewhere.


It would have a 'first love' angle to it, but taken from the viewpoint that the couple didn't actually end up together and haven't seen each other since they were teens.  They are now with other people and many years down the road (not just like a few) they bump into each other again, (either in person or online) and all the 'what if's'  and regret come to the surface. 


They may have an affair, they may not.  Either way would work.  Doesn't have to have a happy ending. 


Beyond that I'm open about what's in the story.  Not a lot to go on but I'd love recommendations of anything that sounds similar.  No matter how loosely :)


I know that if anyone can help it'll be the Booklike crew! Thank you!