If she won't even discuss the issue, I'm afraid you have little choice but to plan your exit.
Ultimately, she betrayed you multiple times. Many people wouldn't have shown the understanding and grace you have by staying with her. Frankly, given this, she should be offering you absolutely anything you need to heal; she's the one who created the wounds.
Reflecting on the Past
Consider these crucial questions:
* What happened at the time of discovery?
* Do you feel she did any work back then to help you move past it?
* Why did you decide to stay?
* Was she truly remorseful back then?
* Did she have any consequences? Perhaps social fall out, or things you imposed on her.
All of these are relevant to your current situation.
Looking to the Present and Future
I'd also ask: Are you certain she's been faithful since? If you haven't gotten to the root cause of her serial cheating, it's generally unlikely that people just become moral and faithful without doing the necessary work. Especially serial offenders. Especially especially... If she had no consequences.
[This message edited by DRSOOLERS at 7:05 AM, Thursday, July 17th]