My girlfriend likes it rough and I don't?

My wonderful and beautiful girlfriend likes sex to be kind of rough. Just normal stuff like biting, scratching, hair pulling, and nipple twisting but I tried it out today and for as rough as she likes it, I have to scratch really hard. I scratched so hard it left welts on her back. She says that she likes the pain and the roughness, but I am so guilty that I hurt her. Should I change how I think about it or what?