Not a fool. If he is doing the MLC thing (affair with the secretary!) then what you do really isn't going to change what he does. Unless it makes it worse. So, your motivation needs to be to take care of yourself, not to try to influence what he does. If you are going to fake it, do it because it helps you feel better.
I guess my point is, don't be too hard on yourself. Everyones's path through this is different, and even if you do everything "right" there are no guarantees. Except that you are a better person at the end. It's not a bad result!