Get her back on the ground, and back to driving, get a solid stop on the horse ground driving, drive her down the fence line, try to get her to stop, when she won't stop, pull a rein the other direction and pop her head off the panel. She'll learn to stop pretty quickly. Make sure you a get a good stop on her before you ride her out the roundpen. Don't pull harder on her head and try and muscle her into a stop either, that will make everything a lot worse.
~Started young horses in Bosca te Ador, unto the two rein the old Spanish spade, brought them along with two hands that were gentle. Some fine reining horses as ever were made~