Sex Problem

I don’t know why.. I feel weird being on top while having sex. I feel like my bf who I love is really good and he tries putting me on top but I just feel uncomfortable or insecure, like I’m not as good as he is. So when I start feeling weird I start to get dry and like I don’t even know how to grind or move or anything when I get on top which is the first time I have this issue with a guy. I have two kids and have stretch marks so maybe that’s why I start to feel shy idk. I want to be able to ride him I feel bad that he does most of the work :( .