I guess my opinion on the matter is that you both made mistakes. If you called it a break up, you both should have been more clear about where you stood with each other. But that is in the past, and can't be undone.

How did the break up happen? Did you break up with her, her with you, or mutual? What caused it?

I can understand why you feel betrayed. Perhaps, in your mind, the break up didn't feel real because of the continuation of communication. But you have to understand that somewhere in her mind, it was real and what she did was not cheating. I'm not going to say what she did was a great idea, but at the same time you can't call this something it's not.

This is going to take a lot of work from both of you. You both have to want to move past this. As long as you hold on to it, keep dredging it up, you're going to feel the pain. Perhaps you can see your therapist more frequently? Or even check out some books from the library that offer advice on dealing with infidelity. I would also suggest journaling and learning to look at you emotions from a distance. Learn to feel them, understand them, accept them, and let them go. In DBT, this is called Teflon Mind. It's going to take a LOT of work, a lot of self talk, reminding yourself that this was not a reflection on you as a person. The others have offered you some great advice.

And I know someone mentioned that your girlfriend can't get tired of hearing you bring everything up again -- but there may come a time when you start hurting her with this, simply to hurt her because you want her to feel as much pain as you do. Don't let yourself get to this point. If you start feeling this way, it is truly time to let go and start healing.