I’m a soft dom but my sub wants harsher play and be a brat

Is not that I’m against doing it harsher, is that just I get in my head and won’t argue with her when she brats out.
Basically she will say no and I’ll just agree with her, or she will say “try me” and I have no way on defeating that.
Spanking does not work cause she enjoys the pain, so I have no ideia how to demand stuff from her.
I also don’t know how to be more aggressive, I get scared of hurting her and she letting me.