OK - You asked for feedback. I have my own experiences and biases here so feel free to blast back. Perhaps we can call this the POV of the divorced parents children.
I am assuming that your X is every bit as bad as you described and I congratulate you for having the strength to divorce him. BUT...
Thread Title - "Betrayal - wounded mom"
I hope you are referring to your X and not your children.
Expecting your children to report to you about your X's activities is unreasonable. They didn't choose to be your children and have divorced parents but you and your X did choose each other at one point in time. If you want to know what's going on ask your X. Ask him how often the children see his GF. If your relationship with your X is so bad that you can't feel comfortable talking to him don't expect your children to fill the gap.
To question weather or not you have a great relationship with your children based on how much they share with you about your X's activities is not going to help you have the best possible relationship with them. I realize this is harsh but IME if you want the best possible relationship with your children don't put them in the middle of the situation with your X. No matter how bad he was he's their father and they seem to want to have a continuing relationship with him. That's normal. Except in the most extreme situations kids want to have a relationship with both of their parents even after they split up.
Again I have experiences that bias me (can you tell?) so please feel free to ignore my feedback if nothing here rings true to you.