Most Helpful Girl
I have never been a mistress or had one, but I don't understand why you would want to be one. As long as he's married and I am guessing still living with her (since you said "at night" you feel worried), he's never going to be able to give you all the needs you should have from a relatioship. From guuys I have know that were married and had girlfriends on the side, they were just that - girls on the side to get away from the marriage life and have "something new" for the sex life. If doesn't matter who cheated first, cheating is cheating however way you look at it and you are just helping him cheat on her (and he could be doing that as revenge because she hurt him when she did it)
Most Helpful Guy
If he cares about you then he will eventually be with you once the divorce is finished. Time will tell what happens.