Join Date: May 2007
Location: Fairbanks, Alaska
Considering her age, and assuming that it is not a pain/fear issue, this seems to be some sort of habit that she picked up from her past. Maybe she was never finished properly or she spent a while doing something high energy like gaming.
I have suggested this to others in similar situations, and will suggest it again. Start her over as if she were a green horse. Put her in a snaffle bit of some sort (loose ring, egg butt, french link, etc.). Teach her to supple up side to side from the ground. In the saddle do a lot of walk/ trot exercises to supple her body and neck. Work on serpentines, circles, figure eights at the trot until she is steady, balance and soft. Work her over ground poles, work on all of her yields, sidepassing, backing, turning on the forehand, haunches.
Focus a LOT on pressure and release to soften her to the bit. Ask very lightly with everything you ask, increasing pressure and contact slowly, and then release IMMEDIATELY and BIG as soon as she responds the tiniest bit correctly. You HAVE to reward her for trying to do what you ask, and rewawrd her to show her she got it right. If you do not release pressure when she tries, she will be less likely to try again because she will not think she was right.
When you have all of these little piece soft and smoothe, walk and trot smooth and balanced - then AND ONLY then give the canter a shot again. At that time, if she rushes, make sure to break the canter up with spirals, figure eights (with simple lead change), serpentines (with simple changes), ground poles, transistions, etc. Keep her focused on what you will ask for next rather than racing. Also, during the canter exercises, the half halts are a good idea if needed.
Half halts for the canter are not a bad idea, but if her foundation truely is not there (which I suspect), it will be useless. It will be a pressure on her mouth that she doesn't understand, that she will fight against. Spend a lot of time back at the basics and working on her bit softness with the new snaffle bit, and then give the canter a try with half halts and patterns.
Good luck :)