Maybe she is poison, maybe a cure. It doesn't sound too positive these days, though.
You had sex while being married. She's been dating, and may or may not have done the same. Forgive her, whether you stay married or not. Forgive and release the anger, the hurt.
I know someone who had a porn addiction. He said that when he told his W (they were beginning to talk D), she was relieved because that had been the big secret he never told her...but she actually knew all along. I took from his story that secrets are deadly. She overreacted to the porn problem - he had been struggling since he was 12, but it was the secret that killed her and their M. He's getting professional help now, and that made their M (for now at least) possible.
Is it possible that the one night stand (or other secrets) have been lurking all along between the two of you? That is why I think forgiveness is so important. Even if you D, and you have many reasons to do it, you don't want to live your life filled with anger taht may have reasons you just don't know. You may never know. You may know the biggest reasons, but the damage may have also been too late to be repaired.
So, either way, forgive both you and her. Accept your sins and hers as past. Then decide if you love her, as she is, enough to deal with all the things she does to make you angry/hurt.
If you do, keep DB'ing and looking for openings. If you don't, move on without her.
I feel bad that you are going through this. I can't remember if you are seeing and IC - if you are, keep talking out your anger to get rid of it once and for all.
