1. You don't know if that was her bf
2. You don't know if that was the Pdoc
3. You don't know if she's sleeping with him
4. None of those things are any of your business
5. She has a right to see her daughter's concert
6. You are not her daughter

Reality sometimes sucks. This is why staying in denial is so unhealthy. Your LCM has her own life. You need to start building yours. I understand it's painful and scary and overwhelming. But it's a necessary part of life and being an adult.

Have you made any progress on finding a T?