"I understand. That still doesn't give you the right to have an affair."
"I hear that you have marital complaints. I'll fully own my share of them. That still doesn't give you the right to cheat on me, and to lie to me and even your own family about it."
"You say that you have the right to do whatever you want. I couldn't agree me, but then I also have the right to -- and WILL -- do what I have to protect myself, and my daughter."
"You try to sound so brave. (laugh a little) It's just ironic, because you really took the cowardly way out, by deciding to have an affair and then lie to me about it."
(If attacked for exposing, specifically):
"I decided that I was no longer willing to lie to cover up your affair."
"I decided that they/he/she had the right to know the truth."
Again, CD, less is more. At this stage, you should really sound like a "broken record" to her -- four or five or six basic themes and statements. When in doubt, these two always work well:
"I'm not sure how I feel about that. I'll have to let you know."
