I see what you mean then about credibility. And it's good that you recognize that you need to be prepared to take steps to enforce your boundaries. That's where I struggle too. It's really, really hard to make decisions we don't want to make or to put ourselves in a position where we might have to. Take your time. Ultimately, if you're not ready to demand that she end the A, no matter what the outcome may be, then IMPO it's best to act like you don't know about her continued contact and try the indirect approach of making her stop on her own, which is what I think you're getting to with your question
what made you feel most guilty? How can I use that to my advantage?
What made me feel most guilty....when my H acted like the H who didn't deserve any harm to ever come his way. See, with affairs, there are aspects of those relationships that led to the A. While no one ever deserves to be cheated on or lied to and hurt.....there are things that led up to it that, for whatever reason, the offended party has chosen to ignore. Stop ignoring those things and be the man she wanted. Not for her, but for you because deep down, you probably know that's the man you should be regardless of what happens with you and W. Be true to yourself and let your integrity shine. I promise you, the guilt will follow.
"Happiness is a butterfly, which, when pursued, is always just beyond your grasp, but which, if you will sit down quietly, may alight upon you."