It's a good idea to go back and work from the ground. Use a halter and lead shank, ask the horse to trot with you and stop when you say whoa, then back up immediately. Once the horse has mastered stopping and backing from the ground, ask the horse to stop when being ridden. When you are riding the horse and ask the horse to stop (always give a verbal command of whoa for the horse to stop) if the horse doesn't stop immediately, ask again and wait until the horse stops. When the horse does stop, back the horse up quickly for a few steps. Make sure the horse backs quickly. This doesn't mean jerking on the reins. Hold her face and use your legs by squeezing to make the horse back up. Release after a few steps. Work the horse forward again and ask the horse to stop, and repeat the backing process again if the horse doesn't stop immediately. Once the horse does stop relatively quickly (not on a dime, but not going through all of her paces), pat her and tell her how good she is and stand her there for a few seconds. Soon she will realize that it is less work to stop. She won't associate backing as being negative when you want her to back. Make sure you reward her when she does stop correctly so she associates that with the positive reward. You should always back your horse after a stop so she begins to recognize that whoa means cease all forward motion and get ready to go backward. A good back makes for a good stop.