I think it's natural to have these problems (I'm not a woman, so I can't say for sure), just because when you begin, you never really think of losing or anything. And it is a sport/competition. Like any competition, you're focused on winning and little things can shake you easily because your adrenaline is rushing. However, time/experience cuts down on this because you learn to be more relaxed and calm. And you learn to take it in fun. You just have to remind yourself that:
A) You're body is fine. You have an amazing body.
B) Her shaking her ass while face sitting you or taking off your top is just her having a little fun or sexing it up for any observers and given the chance, you should feel free to do the same. You shouldn't let it fluster you.
C) Power moves are a part of wrestling/catfighting and to a lesser degree, NHB/fighting. It's not really anything to be afraid of. Unless she's some huge weight lifter, it's all in the technique, use of adrenaline, and leverage. Don't be scared of it. It doesn't mean she's going to begin pounding you. Learn how to counter and try to relax. If you're all rigid, you stand a chance of being hurt more.
D) It's all in fun. Don't be so up tight.
