I have a few suggestions, and I hope they might help you. =D
Try lunging over poles on the ground. If you have any cavelleti, use those as well, when lunging.
Clinton Anderson has some videos to watch. Particularly, watch those on yielding the hindquarters, and backing up, again working the horse first while on the ground.
Yielding to a stop at all gaits (watch his videos, season II for some tips) will help. Watch Clinton Anderson Downunder Horsemanship - Riding With Confidence Free Online
Do you use all of you aids to halt? I was taught when asking for a halt to:
1) Stop following with my body
2) Squeeze my calves to cue "forward", while at the same time
3) Stop following with the bit, which is the same whether direct reining and riding with a slack rein.
I believe, as those that I've studied, the EVERY movement for a horse is a FORWARD movement, even backing.
I also believe in using half-halts. It's a lot like pumping the brakes on your car, and gets your horse to engage his hindquarters without slamming into his mouth, or pulling hard on it.
I recommend that you read this, because his book changed the way that I ride and train.
http://www.amazon.co.uk/Complete-Training-Horse-Rider/dp/0948253517Read this, too: The Complete Training of Horse and Rider