I don't want the kids to be pissed at their Mom. She is the one destroying their world, but, I will likely be the agent of that destruction and I'm not about to be the fall guy in my kids eye's for her lying cheating betrayal of them and of me.
I don't believe that an ultimatum will work. Yet, what is setting and defending a boundary except to issue an ultimatum?
Defending that boundary may mean that I end my marriage, but, my self-respect can't continue to accept her unacceptable behavior. I'm not telling her to get out and I won't say to her that she needs to choose me or choose them. I'll be telling her that I know what's going on and have proof, that her behavior is unacceptable and needs to stop, and I'm going to see a lawyer. Then, I will wait and see what the fallout is of me respecting myself enough to stop putting up with her behavior.