I definitely hear you on this.

Unfortunately, I don't have any good/simple advice for you! I've always been much the same way.... and the only way I've been getting out of it is forcing myself to open up (I started with "easy" hard topics for myself... things I was emotionally cut off from, but that I wasn't comfortable sharing....) and basically repeating to myself that X isn't Y and that I can't treat people as if they're my past people.

So.... I'm at the point where I can share a lot of things about myself more easily, and I've even been approaching the realllllllly hard ones for myself with my current bf. I totally want to bolt each time I try, but I haven't. I just have to keep reminding myself that he isn't my past, and that I simply can't test him - people alllllways fail tests if they don't know they're having one.

You can definitely talk to him about your anxiety and self-esteem issues. I'd personally start with anxiety as it seems a bit more accessible. I know it'll be super hard, but you can do it! Just start small.