Welcome aboard. Sorry to hear about your situation. I have several questions, hope you don't mind. Has there ever been an A (on either side) in the past? If this as fizzles out, will you want her back home?
Do her kids live with her, and are they there when OM sleeps over?
Do you have any idea why she got involved with another man?
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!