Can you get her to canter on a lunge line? Make the circle very large by tieing two lunge lines together (only if they are not the heavy kind) or walking with your horse when you lunge her. It is easier for horses to canter on a curve, and without a riders weight.
I taught my boy to canter by teaching him a voice cue on the lunge line, and then transferring that into the saddle and connecting the voice cue to the normal riding cues.
When you are lunging, do not use any tack or aids. Let him figure his legs and balance out on his own. If the doesnt pick up the canter on the lunge, try lunging him over a small jump. When he jumps (or is landing really), give the cue, and move so that he can canter a whole circle without jumping the jump again. He will drop the canter after a faw steps at first, but just keep practicing ower a few days/weeks until he canters a few circles smoothly. Then replace the jump with a ground pole, and give him the voice cue to start the canter when he is going over the pole. This is when the voice cue is put to a test because he doesnt have to
canter any more, but he should read of your body language and have the balance by this stage to canter when asked to.
Once he canters without a pole, you can start lunging him with a saddle and some sidereins, to simulate cantering with a rider. Just like when riding, do multiple transitions. Once he is balanced a smooth this way, you are ready to try it under saddle.
When you first ask for the canter under saddle, have him on a large circle and in a nice slow but rythmic and powerful trot (you need to feel like he is redy to go, but you are containing that energy). Ask for the canter. If he runs, half halt, return to the earlier tempo and try again. Sometimes it is easier to ask from a walk, but depends on how well the horse understands the voice cue. The first canter will be fast and unbalanced, but let him run. After a few circles, sit down deep and ask him to slow down and trot. If he breaks gait when you ask him to slow down, don't stress, he just isnt balanced yet to canter slowly. You are really just asking him to slow down in order to make the transition smoother. Ask for a slow and balanced trot immediately after the canter -dont let him run at the trot. Do the same on the other side, and then reward by finishing for the day.