Why do I still sometimes feel jealous of my husband’s ex?

I always thought she was a lot prettier than me, but I don’t see why that should bother me the way it does because it shouldn’t matter at all, especially since she’s a crappy person with a bad personality and a crappy life, and my husbands loves me and not her and I have a good heart so why should I feel jealous if I’d hate to actually be her? I never feel jealous of celebrities for being pretty and most of the time I feel confident and pretty myself, but whenever I’m reminded of her I feel ugly in comparison and I put myself down wishing I looked more like her. My husband always tells me I’m beautiful and told me once before that he thinks I’m prettier than her, but I don’t really believe that. Obviously he’s unattracted to her now because he hates her, but I know that if he had met us at the same time he’d think she was the prettier one. But again why do I let that bother me? I hate imagining that he was once with her. They were never in an official relationship, but met at work and it was a friends with benefits kinda thing, but she ended up getting feelings and “loving” him. But she was already married with a kid, which is why she’s a crappy person. Her husband knows she sleeps with other people, but he lets her do it since he doesn’t want to be alone, even though it hurts him. And she stays with her husband for financial security. Eventually my husband ended things with her because he wasn’t okay with that situation. Then he met me and we fell in love, but she continued to try and push contact with him since she didn’t want him out of her life. She even tried to be friends with me for that reason. One day I went to her house since she paid me to photograph her kid for her, and while we were in the car she was telling me how she didn’t want me to dislike her for still having feelings for him, and then proceeded to tell me about her past sexual encounters with him, as if she wanted to one-up me or something. To be fair she didn’t know how things were serious between me and him yet and assumed it was casual, but it was still super inappropriate of her and she obviously didn’t like that he was with me and not her. For months she continued to try and be friends with me, and I was too nice to cut her off at the time, and it was just a really awkward situation. Eventually I finally got the guts to block her completely because I thought I’d stop feeling jealous of her. But it’s been about 5 months since then and she still gets stuck in my head sometimes and I feel a mixture of jealousy and disgust, both hating her but envying her looks. Why do I still feel this way? My husband married ME and he loves me and loves my face, and he hates his ex. Why do I have this insecurity then? I just can’t find the root of the problem. A couple days ago my husband thought he saw dimples in my cheeks while I was laughing and he thought it was cute. I don’t have dimples though, but his ex does, so that was one thing that triggered that insecurity again. That still shouldn’t bother me though, and I feel really dumb that it does. His other exes don’t bother me one bit. I guess another reason that one girl bothers me so much is because I know she still has feelings for him, and by pushing herself into my life she made it personal. Oh, and she actually used to be my coworker years before any of this, so I was actually aquaintences with her at one point, small world lol. I was never jealous of her back then though and at the time she seemed like a nice person. But I just want to stop this primitive competitive feeling I get now that she’s the ex. Advice?