When I confronted my wife the first time, I rehearsed it in my head about 25 times. When I re-confronted her about a month later -- armed with recordings of her and the OM, making fools of themselves and talking about lying to our children -- I typed out what I wanted to say and rehearsed it about 50 times. Out loud, and even role-playing the eye contact and facial expressions that I wanted to use.
Practice it, and then pray for strength. And then just DELIVER IT. I was actually more nervous PRACTICING it than I was DOING it. Remind yourself that SHE is the one that should be nervous, and that has explaining to do!
As someone else just said, tell her you love her (I told my wife that my love for her was a "real" love, that would endure when she turned 50, and went thru menopause, and 60, and 70, and ... she melted and bawled), but tell her that you respect your marriage too much to let her carry on like this. Tell her she has a decision to make, and that you have been patient, but that your patience has an end to it. The line NOP gave me was "I know all about you and ______, and it has to stop right now. You are disrespecting me, you are disrespecting our marriage, and you are disrespecting our family."
You can do this. And you will feel SO much better for having stood up for yourself.
