It does sound like an opening. LOVED how you handled it -- way to go!!! I would meet with her, and just listen. Let her go first. Make no promises; expect nothing, give nothing.
If, in her speech, she says she realizes she must end it, let her finish and then say "OK, if you're serious about ending it, this is what we need to do," and then have a plan ready for her to write a "no-contact" letter and to put a "transparency" system in place.
If she waffles all over the place, and her speech is full of "I'm confuseds", and "I need some space's" and "I just don't know," then hold her hands and say "I'm sorry you're in such a confused place, and I want to help you. But until you end it with him, there's nothing I can do other than tell you that I love you, I DON'T want to divorce, and I stand ready to work on our marriage as soon as you end it with him."
If she at all approaches anything D-related, simply say "It would be best if we let the lawyers straighten all that out."
I would NOT get into any self-analysis and self-criticism with her. If she insists on you telling her what makes you happy, frustrated, etc., just say "Nothing would make me happier than for us to grow old together, and our grandkids to be surrounding us with their love; and what makes me angry? The idea that you decided it was OK to invite another man into our marriage and break your wedding vows. THAT makes me angry." And leave it at that.
Let us know what happens.
Choc.
P.S. This may sound hokey and stupid, but don't be afraid to PRACTICE the things that you want to say, especially if they don't come naturally to you. Role-playing (when no one is around to look at you like you're wack-o, of course!) can really help make you confident in tense moments. It did for me!!!
I would meet with her, and just listen. Let her go first. Make no promises; expect nothing, give nothing.