This will be short and sweet. The above post was entirely focused on your XW. My only advice: She is going to have a period of time after breaking it off with OM when she is down. Stay out of her hair, don't analyze her moods, and have as little contact as possible with her. And don't give me reasons why that isn't possible. You can pick up the kids and go to the gym with your step-son without going out of your way to talk to her. A perfunctory "hi" should be sufficient for the next couple weeks. Let her process her life since it's bound to be different with OM out of the picture (if he stays out).
In the end, it's not the years in your life that count. It's the life in your years. Abraham Lincoln
It is hard to fail, but it is worse never to have tried to succeed. Theodore Roosevelt