What you have with OW isn't love, it's lust. If OW is cheating to be with you, she will cheat on you. Of course her M is on the skids. She only married because she thought you weren't available. She seems to take M very lightly.
Lose the OW and make your M what you want it to be! You don't have to settle, make it a good one. Read "Divorce Remedy". Read "The Five Love Languages" by Coleman. Go see a marrage counselor. (But make very sure it's a really good pro-marriage one, preferably Solution Based, Brief Therapy.
The major ingredient in making a marriage work, is wanting it to. You can bring back the passion and romance in your M. You just have to want to.
Instead of thinking about what you want, think about what your W needs and what would make her happier. If she is happy with you and the R, I can almost guarantee that it will be a better M for you also.
Try it before you give up. The best gift you can give yourself, your W, OW, and her family; is to walk away from that R. Work on your M and tell OW to work on hers. I think you will all be a whole lot happier in the long-run.
