The "Mr. Nice Guy" approach isn't going to work. I know it's most of ours go-to move, but all it's probably doing is making her lose respect for you, and it KILLS attraction.
And right now, you need to be re-BUILDING attraction, not killing it.
Leave her be for the most part, but do establish some boundaries that work for you and the kids (10pm is not late, but she shouldn't stay out past midnight, for example). Let her know that you do NOT want a divorce, but that you're not willing to live in an open marriage, either, and that her friendship (if that's all it is) with this guy is not appropriate. Acknowledge where you've been lacking as a husband, and your determination to work on those issues and keep your kids' family intact, but you're not going to be her second choice while she goes out at night, and that you BOTH have some thinking to do and decisions to make.
And then let her be. Remain a little mysterious, positive and upbeat but a bit aloof, and DON'T PURSUE HER. Smothering her will push her away, and she likely finds it weak and repulsive.
