I hear you. The awful thing is you can't stop her from going back to OM (or finding a new OM.)
The best I've been able to do is lay down a boundary that when we do get together to talk she is NEVER to bring up or discuss OM or other relationships with me in any capacity. My W has found herself in an emotionally abusive relationship with OM (and still is I assume), and expected me to be her coping mechanism while STILL dating OM!
Don't be an emotional crutch. If she needs an ego boost or reassurance she should see a therapist and work on herself OR be faithful to you and get the emotional fulfillment of a healthy marriage. I'm guessing she's probably not reasonable enough to understand this so she'll probably have to stumble around in the dark and scrape her knees for a while first. That can be tough to watch when you love her so much. So GAL without closing the door on your marriage.
Age: 28 Wife's Age: 28 Relationship: 10 Years (dated on and off) Married: Less than one year Seperated: 1.75 years, finally served with D 4/30/2011