How well does therapy work? Someone with psychology experience answer if possible

Hi, I wrote before on how my boyfriend of a year kissed another girl and then wanted to kill himself after I broke up with him. What has happened since is that is he is actually going to therapy over this. He is trying to figure out why he is a cheater, and how he can stop it. She thinks that it is a combination of him being cheated on by every girl he's been with, (except for me, mind you) and that he is going through a phase that can be corrected. She thinks that it is also attributed to that he didn't get enough attention at a critical part of his childhood. He is bound and determined to work through this issue. He would always say how therapy was useless, and badmouth it, and now he is in it because I told him to go. That shows something. I am hoping that it will really work for him. If he finds the root of his problem through therapy should he be okay?

And another thing is that even if he is "cured", I do not know if I will be able to take him back. I feel really betrayed and I can't get the thought of him kissing another girl out of my head. I don't know if I can ever trust him again. He has also suggested relationship counselling. Would that help? Keep in mind that my family, especially my mom, is against us being together.

I know that I've asked a lot, but any thoughts or help would be really appreciated