I believe that you have to work at your marriage. But I do not believe that anyone should have to work so that their spouse will not be unfaithful. Thats just not a fair ultimatum in a relationship.
And that's kind of what it becomes when a spouse serial cheats. Stbx. Your spouse is not remorseful he does not want in the marriage. He does not want to work on it.
Looking hot, not reacting, being pleasant and friendly when he's trying to take advantage is not realistic and you set yourself up for failure.
Get dressed up and go out and do things because it will make you feel better. But you have to let him go and all thoughts of him returning as well.
Him and OW deserve each other. He's with her because the last OW ended things. They won't last. But don't be waiting in the sidelines. You deserve more.