He's clearly not over the split. But he should control how he behaves. Document what he says and does, and give the documentation to your lawyer, and have your lawyer talk to his lawyer about the fact that he needs some therapy or anger-management counseling.
It' sounds like you're right. Your ex is not fine with the children if he is belittling to their mother, in their presence, or even without them knowing. His constant abuse has a consequence making you feel bad a bout yourself, and having to deal with that affects your being at our absolute best. He is sabotaging his kids, by sabotaging their mother. It sounds like he doesn't see this connection (which means he's not terribly smart) , but is trying to be a half decent decent father figure, but not an exceptional one His having i"issues" with you, trust issues, or any other, he likely has intolerance of other people, and that's not an ideal role model, so he's not a perfect father by any means
It' s a lawyers job to present a case for parental alienation, i don't know if you're able to get a lawyer. It would go to court, and he would be told that he could no longer say or do anything that could be considered parental alienation. If he was unable, custody may be given to you. The stakes are high enough to warrant that kind of intervention, if he would not listen to reason ( you could also try to appeal to his sense of fair play in a letter) Your relationship with your kids could be sabotaged, your children could grow to be disrespectful of their partners, or accept disrespect in their relationships. It's so sad.
You ascribe "trust issues" as a possible reason for his disrespect for you. Was he raised in a co-dependent home, or was he cheated on by a previous spouse? If you don't want to get a lawyer involved, you might want to talk to a therapist, IT might be that you can talk to a therapist and find the most effective way to bring up the subject of you both supporting each other to most successfully co-parent your children. If you went by way of a lawyer and court proceeding, it might be that you can push for therapy with him , so that you and he can successfully learn ways to most effectively parent.
My heart is with you. It must be so hard. It sounds like the kids stay with him, and that usually seems to happen because of a man's ability to make more money, and having more money does not necessarily mean that you are the best parent to have custody.
Have you considered fighting for custody of your children, based on his actions? Even if he doesn't belittle you in front of the children now, doesn't mean that he doesn't behind your back, or will not in the future, resulting in the same potential hazard for the kids.