Everyone has given you great pointers on the stirrups and on how to post. I think what you need is a horse that has and maintains a good rhythm at the trot. Lots of gaming horses are inconsistant at the trot because they never needed to work the trot that much. My horse definantly needed some work on his rhythm!
If you can ride in a two point, its best. Get your horse moving and look for that one-two beat you are looking for. I try to think of music when teaching this...or a ticking clock. Get up in your two point at the trot and try your best to stay off your aids and her mouth. They will find a rhythm they like and stick with it. Once she settles in to a beat drop yourself back and try to post. It may be that she's just not used to a rider posting! Lots of western horses don't understand it and can sometimes confuse it with a cue to canter or in your case, a cue to stop.
I think you just need more time. I am no professional but I know that posting a horse without rhythm can prove to be very difficult!