I feel the urge to confront it with her. At least if I know for sure she is dating it might be easier for me to let her go.
If she chooses to lie, then you still won't know. Even if she admits OM, it probably won't make it easier to let her go. Many H's have said an affair would be a deal breaker, only to discover it wasn't.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!
