It sounds like you need to work on your balance and movement with the horse. When you canter, you want to move with the horse, but not "pump" as they call it. (Moving too much with the horse's canter.) Then it just looks weird. Make sure you are balanced in the center of the seat and the horse. When they first pick their front legs up to go into a canter, you want to lean forward just a little
, then when they come down, you want to lean back. Continue to do this with your mid section, but not your whole upper body each time the feet come up and then down. It sounds weird, but it will actually look fine. (This is the best way I can describe what I'm doing when I canter.) I think you are leaning forward the entire time and that is what's causing you to lose your balance and come off. Also, squeeze with your inner thighs. This will help with balance as well. It all sounds complicated, but when you get into the practice of moving with the horse, it will look harmonized and natural.