Jen, I'm glad you and he had that talk. A lot needed to be said and I'm glad you both ere able to say it and also to hear one another.
Ongoing things you have to deal with (and probably accept) -
1) Ex is going to keep on as she is. She has no need to change. He is in the habit with her of giving way and gelling her stuff, habits like that are hard to break especially if you don't know why. So I think you need to accept that he will continue to talk and text her, and to tell her stuff he shouldn't. You need to keep your cool, stay open and relaxed about it and then he is more likely to be open with you about what he has said. And you at least will be informed and not feeling so much like a mushroom.
2) You need to not try to control him so much. If you have to keep a tight leash, you will never know if he stays by your side simply because he can't shake his collar loose. When he stays by your side without benefit of leash and collar, you know he is yours. You both need to work on your insecurities and learn to be honest and open with one another. This is something you need to work on every minute of every day, and lately difficult child has totally distracted you, made you needier and more insecure as well as added to your burdens. You can vent here; he vents with other people. And he doesn't vent in healthy ways. You can work on you, but don't make it competition with him. "I'm trying real hard, he hasn't changed a bit. Why should I bother?" Don't do that. Just put one foot in front of the other each day, keep walking in the right direction.
3) Things have changed for the worse thanks to your kids. He's not well equipped to handle that. Who would be? But at least they are YOUR kids, you have tat reason to fight on. It's more difficult for him. yes, he knew about it but didn't bargain on it getting this bad. Your feelings have changed - right now you're tired and stressed, feelings do shift and wobble and a long-term relationship is generally worth fighting for.
Hang in there. Hopefully the therapist will follow through.
Marg