I really feel Spyder and Anabel would beable to help out most definitely here.
Thanks for the vote of confidence.
Couple of things going on here with the OPer's horse. Yes it is a balance thing and part is in the canter itself and part is in the transition to the canter.
The better the gait before the canter the better the canter will be. It is HERE where the half halt will be most useful. Try strating the canter in the shoulder in position and as soon as the horse becomes heavy stop cantering and re set up the trot to canter again. The longer you ride the horse when it is not right the more you are training it to be wrong.
Also it isn't just doing or not doing the half halt but where it is done in the canter sequence. So many people give the half halt when the horse is in the down position (the forehand is taking the most weight) but this is the wrong time for it makes the horse even hevier on the forehand, and going behind the bit or too low a head carriage is the usual result. It also doesn't hurt to simply drop both reins for a stride or two and re pick them up so that only the barest contact is being had.