How do I top dominantly?

My boyfriend and I are kinky af. We have a pretty set rhythm though and are trying to spice it up, which I'm definitely down for but... I have trouble doing what he wants me to. What he wants me to do is top, and not only to top but to be dominant while topping. I dont really know how to do that though because when we do stuff I am almost always the bottom. I love to sub but I want to learn how to dom for him and I know I CAN dom if I'm in the right mood, I just dont know what to do. I also dont know how to do it without getting him nervous because whenever I get close to being able to do it I have to stop because he gets nervous and idk how to calm his nerves. Any advice?