Who said to tell her to "F off?" You should treat her with civility and courtesy -- she is your wife, and the mother of your kids.
btw, is your 12 year old a son or a daughter? This is a VERY formative year, these next few, for how they are going to shape their views about members of the opposite sex, and how you allow yourself to be treated. Don't think he/she isn't watching you.
I'm sorry if it stings, but I'd rather see you maximize your chances of success. Even the right way isn't guaranteed, of course, but I can pretty much guarantee you that the way you're doing it isn't going to work. What does your "gut" tell you? Does it feel authentic to your own values and morals to ignore -- and even enable -- her affair?
I would start with not allowing yourself to be disrespected by her talking about -- or calling/texting -- OM in front of you or the kids. That's #1. Then I would look at is the family's finances being used to conduct her affair? What are the current financial sitation? (who pays for what, etc.)
There's NOTHING that says you can't change your mind. And when you do, and when she acts surprised, just say "You know, I did a lot of thinking, and I've decided that this isn't working for me anymore." And go from there.
