Help! Can't get over the past

I (F28) met my husband (M30) on march 2020. The relationship moved fast but it has been amazing and every step of the way has felt right.

There is only one problem: his ex. He dated s girl for 8 years and broke up with her a year before he met me. They are still good friends and talk every now and then because they have a lot of friends in common, which is perfectly understandable.

The thing is I don't like her and she doesn't like me either. When I started dating him, she still thought they were going to get back together and she didn't really hide it. I understand it was painful for her but she was really rude to me. We haven't really hang out with her in the past year because of this and he has had less contact with her because I asked him to. During the time they have not been speaking as much he has started to realize how toxic his relationship with her was, although he says he doesn't really mind now because he is with me so everything turned out fine.

The problem is that I don't know why it bothers me so much that he has a past with her. Like, when he tells me about things they used to do together I feel like he wants me to be like her or that he is not really happy with me and I don't know why I feel like this. Her and I have some things in common, like the fact that we like baking, and I don't know why that bothers me so much, but it does. It's like I feel he wants me to be a replacement for her or that I'm not special to him.

I feel like I am going crazy, I get super anxious and I feel like I want to cry whenever these things come up. I am scared that this anxiety might break my marriage.

My husband and I have really good communication, normally, but it is so hard to talk to him about this, specially when I don't understand it myself.