Are you absolutely positive she's not having back/stifle/hock issues? Sometimes those are hard to see, especially if it is bilateral. Horses that refuse to stop or back up and are spooking more and more almost always are telling you they hurt somewhere. Does she get worse the more you ride her? Check your saddle fit. Is she hollow behind her shoulders? Palpate where your saddle fits, especially behind the whither, does she turn her head quick and/or swish her tail? Move away, pin her ears?

Also, pushing the issue on one that refuses to back up could get a dangerous protest and you could get seriously hurt. Have you tried line driving? Much safer to do these things from the ground first. Going to a shanked bit on one that has not had the training to carry one could also get you a dangerous response. It would help if you could get some pics up of her without tack. Need to see a front, back and profile.