If you're more comfortable with being a bottom and submitting, perhaps that is the role you should stick to. Forcing yourself into a role is usually not a good thing.

If you're usually in the submissive position, just try doing a few fierce things and work your way up; like if he's on top and you two are going at it, try digging your nails into his back or biting his shoulder a little, get a little rougher but stay in your comfort zone so you don't freeze up.
If it's more of a deer in the headlights kind of thing, like you're self conscious of him looking at you, then try blindfolding him and having your way with him while he sits back and enjoys it. If you tell him that it's your turn to play with him, then I'm sure he won't argue

Something I do to switch roles between dominant and submissive with my boyfriend, is to pretend I'm in a different persona; like an entirely different person. Thinking differently and acting boldly based on a role model helped me.

Just experiment until you find something you're comfortable with

Try being creative with the domination aspect. For example give him the instruction to stay in a position and not move. Also watch his reactions. If he is really enjoying the scene you made for him it can be an adrenaline boost for you. After all it feels good when your partner really enjoys what you have done for them.

Also choose role playing in which it is easy for you to be on top-like a teacher or a nurse or whatever strikes your fancy. Is there a movie or sci-fi series you enjoy together?

Getting costumes and props and decorating the room a certain way can help you get into character. Perhaps have him dress up a certain way for the scene. If you tap into your creative side this can be a lot of fun for the both of you.