Well, I don’t think you even realized it but typically when referring to a D/s relationship, most often times people are talking about more than play, but rather a there is a power exchange in the relationship (submissive grants power over to Dom; Dom assumes a power shift). This can be in a multitude of ways.
Regarding what I think you were asking – your girl likes it rough but you are struggling with how not to hurt her, I think (a) you’d be surprised how rough you can be with actually “hurting”, in a way she won’t want, and (b) most likely if she has a masochistic side, which many subs do, you will find that she wants a level of hurt.
My tip, which is how I personally approach this, is first a woman’s vagina can take a beating so be a bit more rough than normal (ie when fingering bee). No matter what, make sure she cums a lot. And stick to hands around the neck, slapping her ass, and pulling her hair, then move into a bit more aggressive stuff if it feels right for you both.
Also – read up. I’m in the middle of a book called The Loving Dominant that I recommend reading. It’s geared towards Doms but has value for everyone and in some circles is seen as “the” book to read. If anything, it should help get the neurons firing.