Recently, I've been in the same kind of situation. But I solved it nice and easy. You just talk to her about it. Talk to her how she ditched you, or maybe she doesn't deserve to be with that guy. I did the same exact thing, and I realized I deserve way better. Currently, we are just friends and we both agree on it. So, my advice to you is to talk to her about it, and see if she is worth it or not. And, it was not easy either. It actually took me multiple tries to get it sorted out.
-Spawner