This is my first time doing this, but here goes:
I don't remember the name of story or the author, but it's an AU that started with Dean trying to jump off a bridge. He's saved by Cas and taken to the hospital, where Cas is assigned as his official suicide watcher (I think Dean lives in an old library or something but I'm not sure). At first their relationship is super rocky because Dean does not want Cas there at all, and when they actually fall in love Dean pushes Cas away. Cas dates Balthazar for a while. Eventually Cas goes to visit his family and Dean sees this news report about his plane crashing so Dean ends up committing suicide by jumping off the bridge.