I had a horse that I bought a few years back that would not stand still when stopped, although he stopped nicely, he just would prance around. He was a very high-energy horse and had never really been ridden outside of a round pen much, so I didn't blame him for it.
After a month or so of allowing him to do this while I was getting some miles put on him, I was starting to get sick of it and decided it was time to actually fix it once and for all.
So what I did was on one of our regular 4+ miles trail rides like we did almost every day, I made him STOP about every 10 steps. No joke. And he was not allowed to start walking again until he kept his feet still until ***I*** said he could walk again. If he would move before I asked him to, we backed up. Then I stopped him again and gave him the opportunity to stand still. If he still didn't, we backed up again. When he would stand calmly, I would squeeze my legs for him to walk and we would proceed ... for the next 10 steps before we stopped again.
That was a long ride but by the end of it and ever again after that, he stood as still as a statue when we stopped. And he didn't move an inch until I asked him to.
Of course, just starting out in the ride, I'd only ask him to stand quietly for 1 or 2 seconds to start with. I slowly increased that as the ride went on until we were standing still for minutes at the end of the ride.
You can apply this same concept with getting your horse to stop. The next X number of times you ride, just ask her to stop in random places for no reason. To cue for a stop, you should first "stop riding". That means sink your weight into the saddle, make sure you are not using any pressure with your legs (since that means "go"), and say "whoa" in a firm loud voice. If she does not respond to your seat and voice cue, THEN add in the rein cue. Pull straight backward evenly on both reins. You do not have to pull super hard, just keep the pressure steady. Do NOT release any of your cues until she comes to a stop. But the very instant she does stop forward motion, you need to immediately release the rein pressure and stop saying "whoa". By releasing the pressure, that is her reward for correct behavior.
Note that you can teach this from the ground FIRST as well. Work on her ground manners when leading and use the "whoa" cue and the lead rope.
You can reinforce the stop cue by then backing her up.
In the beginning, it is okay if she does not stop perfectly or if she does not stand still after you are stopped. Work with small goals first and only progress to larger goals when she is consistent with the smaller things.
She may fight the bit or try to pull back on you. YOu do not need to pull harder or jerk or anything like that. Just hold the cue steady until she responds correctly. But just remember to reward her instantly when she does something right.
It might take you 10 seconds to get the first stop or 10 minutes. It doesn't matter. Stay consistent with her and always end the lesson on a good note.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.