Op, if this is real, which I hope it isn’t, then you are deluded. But with that, I’m afraid it means that nothing we say here will make any difference.
Your ex has moved on. He has a long-term relationship with another woman, who he loves. He loves her and wants to put her first, hence he is starting to go against your usual expectations.
He owes you no loyalty. He can be a good dad to the children without your involvement. He can have days out with the children and his new partner, excluding you, and he would still be able to be a good dad.
His partner has been very patient, understanding how hard this must be for you. I’m guessing this from the fact she’s apparently put up with it for so long. But this is not sustainable for the future. She is now asserting the fact that she is a presence in his life.
How would you feel if you met a wonderful man who you fell in love with, but we’re excluded from all of his family events because of a deluded ex partner? She is a human being and deserves better than this. Your ex needs to get a back bone and tell you where to get off.
Whatever you ‘decided’ when you split was unrealistic and unreasonable. He does not need to be with you, involve you or be part of a family with you, to be a good dad.
Op you need to get over this. It’s not healthy.