In my long experience the best thing for a guy to do in your position is to take the aggressive stance..
YOU file for divorce and push this forward now. YOU come across as if this is now what you want.
Men who stand up to this garbage and take a strong stance are the ones most likely to succeed at this. Deep down a woman respects a man who takes decisive action. She secretly knows she is wrong.
Tell her this... "I have been doing some thinking and here is what I have decided. I have decided that you are right. This isn't working. I agree that we should separate and divorce. I think since you are having cyber sex and probably even an affair that you should move out. The sooner the better. I would like you to find a place to stay within a week."
This is said firmly and matter of fact. Not mean. STRONG and decisive. Don't tell her you love her and don't tell her you want to work on the marriage. None of that usually works. Don't let her cake eat. Send her packing and thinking she is the one who screwed up.
Tend to AGREE with this approach. I'm just trying to learn a little more about the sexual dynamic between the two of them, to make sure there wasn't any neglect or other serious issues, and also gather a little more intel. But yeah -- this is the best approach to take.
