If she says she wants a divorce, you say "I can't stop you, if that's what you want to do. I'll be getting some legal consultation then, so that I can understand what I'm up against. I don't want to divorce. I think we can work at this, but you need to end your affair immediately, and then we can get some good marriage counseling."
If she tries to have a relationshp talk (including anything about "we were never right for each other," or any other "re-writing of marital history"), listen to her, and validate her. And say "I agree, we have some challenges. And we can discuss, and work on, whatever you wish, as soon as you end your affair."
If she asks for "space," tell her "I can't stop you from leaving, if that's what you want to do. This is my home, and these are my kids, and I'm not going anywhere."
