Honestly, it seems shes either lying to you or herself....she needs to decide what she is.
Either shes your sub or not, sounds as tho she doesnt consider you her superior. Is this what you want? Do you exude the needed confidence to make her feel your Dominance? Are you her Dominant.....all legit questions, only you can answer.
It seems you are not physically capable of kicking her ass, or at the least dont desire to. Understandable.......again I ask, do you want to HAVE to do that to get her to submit? If so, get your ass in the gym so you can.
Personally, I believe a sub/slave should submit voluntarily, not need beat into it. Although, if the Couple/Group decides thats best for them, then enjoy. I just dont read that from you.
Short of physical Dominance, mental Dominance is all We have, therefore, I revert back to her wanting to submit.......if shes not willing, how can you make her.....this is not to say, there are no challenges, or 'bratty' moments.....not at all.....
I see the real question here as: What type of relationship do you want? One of constant turmoil, fighting, and physical confrontation? Or one where you are the Dom, shes the sub, and you 2 grow together?
Just My 25 cents....