Now is the time to really work on you. She behaved badly and that's all her. But you have to own the part you played. You have to figure out why you accepted such a negative relationship back. She obviously is a cheater but you accepted her back. What was the payoff. There is a difference between not thinking you deserve better and being too gullible. That is where you start. Why did you take her back. You find the reason, you find the problem. You should take some time to do some soul searching. You may just have to big a heart but use this time wisely. You can then figure out why you would take that chance. You can move forward by understanding this relationship. I would explore the quality that allows such a relationship and really begin to move away from negativity. Find a therapist in your area or take the self help avenue but work diligently on understanding what you will allow for yourself. It sounds like you trust others maybe a little too much. Either way you would benefit from working on the kinds of relationships you allow in your life. You do not cut off relationships cleanly and soon enough. These are called boundaries. Learn your boundaries and not allowing someone to cross them.
Please press accept; this is the only way I am compensated
I hope she did too. You seem to have learned a lot and that is all we can really gain from this situation. It is positive that you realize that she is his problem. Let them have each other and next time try to instill boundaries early. You did nothing wrong so it is kind of sad.