I don't like to rely on the horse being tired before they will be obedient to what I am asking.
If there is one thing that I have a NO TOLERANCE policy on with my horses, is stopping. When I ask for a stop, it should happen immediately with no questions asked. Period. I don't care where we are or what we are doing, but they will stop when I ask for it. I think it's just downright dangerous if they don't.
TexasGal has perfect advice. You need to always perfect things first at the walk. When you can get a nice clean stop 100% of the time at the walk, then try it from the trot. Don't put yourself into a losing situation by asking her to trot too soon before you're ready to get a good response. And if you can't keep her at the walk only, you've got other issues to deal with too.
Also make sure you are asking correctly in the correct order.
Starting with the walk.....
Make sure you are sitting up straight and tall while she is walking.
When you are ready to stop:
1) Sit down in your saddle.
2) Put your weight into the stirrups (as if you are pushing on your stirrups in front of you)
3) Say "whoa"
4) Put light contact on the reins, evenly on both sides.
Hold until she stops softly.
The very instant she stops, you need to immediately release ALL cues.
If you don't, she'll learn to ignore you because she's not getting a reward for doing something correctly.
You've got to be consistent. If you let her "get by" with something one time, or you don't cue the same way every time, it's going to be a lot harder to get a correct response.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.