A light ESH. I get where you're coming from and you have some legitimate complaints, but your mom is not betraying you by helping your ex with her grandkids. Especially since it sounds like he's gone out of his way to develop a good relationship with her.

So yeah, you are TA for your main issue. She is prioritizing the children over her child, and I get that that hurts, but you gotta focus on what's best for the kids, even when it hurts your pride. The strength of your reaction to this situation suggests that you are not yet done grieving for this relationship.

However, this is a major asshole move on your mom's part:

She also has a habit of telling me about the things he and his girlfriend do, as she would when talking about any of her friends. For instance letting me know how nice his/their place looks now and what kind of interior decorating projects his girlfriend wants to take on. She doesn’t understand why this is inappropriate when I tell her not to say these things to me because I find it hurtful.

There's keeping out of it for the sake of the kids, and there's rubbing it in. This is an entirely reasonable boundary and it's pretty cruel of her to keep bringing it up.