If I were you my goal tonight would be for her to walk away with the feeling that something snapped and you got fed up with her crap, have no interest in being with a cheating vow breaker, and are totally on board doing anything to facilitate the necessary steps to remove her from your life as a spouse. That you will be fair, solid, respectful, and make sure the children will feel loved and safe from both parents, but that the sooner she remarries OM the sooner she'll be his problem and not yours. You don't have to feel that way, you don't have to try to send that message...but that's the mindset that should guide your steps.
OK, this isn't quite the right mood. Better would be that you realized that you two can't stay married and that you two aren't compatible. Replace my feelings towards cheating spouses with their rationalization on why they are acting like they are. You can still feel the way you do, just make sure not to make her feel judged or criticized. That's non-validation. You can validate without agreeing, so that means neither approval or criticism. Just moving forward and getting this put to bed. Neutral. Stay neutral emotionally. I hope that goes without saying, but wanted to catch myself there.
Me:38 XW:38 T:11 years M:8 years Kids: S14, D11, D7 BD/Move out day: 6/17/14, D final Dec 15