Over thinking

When my boyfriend is in charge during sex we have a great time, but he wants me to take charge more. However, when I'm on top, I start over thinking what I'm doing, and he can tell (he says I'm distant). I love sex, but being in control doesn't come naturally to me. I worry that I'm going to do something wrong (I don't know, just something). How can I worry less during sex and be more comfortable just letting go, without overthinking what I'm doing?