Ah op I've been in similar shoes (no kids thank goodness) but I have to say I think you're being unreasonable here. As far as you know (from what you've said) they started dating after you both finished. I'm sure that felt like a major kick in the teeth and you were left questioning why couldn't he be that person for you and your kids. That's shit.
But as others have said this isn't about you and your feelings as valid as they may be. This is about enabling your child to have a relationship with their dad instead of making it difficult by refusing their step mother to be present. At some point your children will grow up and have a relationship with her that's independent of you, they may want her at their weddings/graduation/birthdays and OP it will unfortunately be your job to suck it up.
Many men have done much worse and there will be lots of women on here who have had to grit their teeth and swallow it for the sake of their children's wellbeing and then vent to their friends with a glass of wine after the fact.
You say you're not jealous but when you talk about the comparison it sounds like you feel as though his life is better than yours in some way. So my advice is to focus on that. Work on improving your relationship with yourself, treat yourself to a nice new haircut or whatever would make you feel better and try to work some time into your week for your own hobbies and interests.
It sucks for you that your ex is now a more settled person, but that is better for your kids than him being an arse who's out every night. What your kids need is the best version of you as well and when you're pouring your energies into trying to block their step mum all they get is the bitter version of you.
In answer to your question I think you've put him in a shit position by insisting that she doesn't come. And he's felt that was the only way to draw a boundary with you which is fair after such a long time. I agree with the other pps who have suggested therapy. I did it after my ex and I broke up and it was important in helping me heal from that hurt and unfairness.
And just to reiterate noone is saying this is fair. Or that he treated you the way he should have. We're saying you need to rise above and be the better person here so your kids aren't left dealing with the animosity.