Does anyone else feel intimidated by their partners ex?

I feel like I might be intimidated by my partners ex. We are heavily involved in each other’s lives because they have children together. But I feel like I have become the frumpy one, and she has become the one that fixes herself up and stuff and it used to be the other way around. It’s slightly depressing. And she makes me feel small and child-like. Like she is my mother almost. It just makes me feel sad and yuck. I don’t know. I’m just venting