Continue therapy, possibly take some individual therapy yourself. If you cant get over it than you probably need to let her go and you both need to move on.
I have had this same issue in a relationship before, stuck it out for years to not ever get over it. It festered inside of me and caused more and more anger towards my ex g/f and the situation. It was not healthy and our break up was a horrible one. It was a very unpleasant situation. If I could go back I would have left as soon as I found out because I knew I was too insecure and anxious to let go of what happened. Insecurity in self plays a huge role in not being able to let go of the situation.