"His dad put the hard sell on him about how the gf didn't have anything to do with it"
This IS NOT a hard sell. The A, the R with GF is a RESULT of the problems in your R. Quit trying to lay it on their R. For your own sake AND your kids, let go of that. Your M ended because it did not work for X. Nor did it work for you, as you are getting closer to finding. It was a sick and limiting R. That is why it ended. That is why X's R with GF started. X took advantage of a vulnerable woman (GF) to make his transition easier, but it was not the cause of the transition.
