I have never been in this situation, so my advice here is on the theoretical level only. Plus, it would help if you gave more information like why you took a break, how long you have been together, what kind of break it was, etc.
If you don't break up with her, I think the best bet for you would to not get mad at all. Keep your feelings to yourself and pretend you don't care while intimating that perhaps you did something similar (like fvcked your ex) over the relationship break. You were, after all, not going out at the time, so its not like she did anything wrong. Depending upon how you display your anger, it may show jealousy and insecurity.
You say that the tail is good, then here is an option. Try to "emotionally next her". Like, stop caring and cherishing her and all that but still keep her around for the tail. Emotionally cold? Perhaps. But if you are sick of her shiet, this is the best protection for you. Besides, if you guys took a break in the first place and she fvcked her ex (depending upon the circumstances) I don't see good things in the future of your relationship. You are probably destined for a break-up anyway.
Still, I think the board needs more information in order to offer better advice.
Peace.