I do feel for you OP. I can see how much he’s hurt you, especially saying he’d never marry when you had kids together, then quickly marrying someone else.
When parents have an unfriendly split, I think it’s more sensitive to attend school events without a new partner, as it can cause stress for both the other parent and (more to the point), their DC, if the new partner’s there. So, I do realise how insensitive it feels that he insists on going with his wife or not at all.
But overall, I agree with @EdgarAllenRaven. By you insisting that he should come alone or not at all, your DC are missing out on their dad being there. And it’s so much easier for kids if their parents can try to be friends, as well as being friendly with new partners. It really affected me that my parents never spoke again until the day my dad died. It made me feel pretty worthless they couldn’t be friends for my sake. It caused my sibling a lot of anxiety.
Don’t let his wife’s presence bother you. Don’t compare yourself to her. If you can now see the failings of your ex, and what an unsuitable/untrustworthy partner he is, don’t you feel lucky you’re no longer stuck with him and just a little sorry for her?
Hold your head high. You know your worth, as a partner and a mother. Your kids will only ever have one mum, and that’s you. No one can take that away.