Here's the thing: after you break up and you realize how much better things are with them gone, you really don't want them back. It doesn't matter if they've morphed into everything you've wanted them to be, you're just happy they're gone. They turn into a bad memory. Seeing them reminds you of your unhappy times together. Nothing they can do positive in the future will ever erase the fact they made you hurt and treated you like trash in the past. You move on.
I wouldn't hold your breath waiting to get your girl back. You might, if she's young and still believes you have the capacity to morph into a totally different person, but most girls won't bother. The hook up may have just been that. A way of connecting with old feelings without having the responsibility of a relationship.
The reality is, eventually they realize that it's much better to find someone that treats them good consistently, than try and fix one that didn't.