It sounds like you need to go back to the beginning with her. Start on basic respect and ground manners first, teach her the verbal commands as well, whoa MEANS whoa, ect. Do some ground driving as well without the cart behind her, and work your way back up the the cart. Find something else like a lighter tire for her to drag around, to really get her to accept listening to you while pulling something. It sounds a bit like she's maybe scared of the cart, and trying to just run away from it, or someone let her get away with ignoring the handler and doing whatever she wants, so now she does whatever she wants. A harsher bit isn't the answer as you discovered. Is there someone in your area experienced with driving horses, that could just give you some pointers, since I'm not there, and can't see exactly what she's doing, I can only give you general ideas of what it may be, and what steps to maybe take. Good luck with her.