This is a very common problem for learners. I still have some trouble , if the horse is reluctant to take the canter. I , and many folks, tend to lean forward too much, which usually results in a faster, strung out trot. That makes it hard for you to keep your balance, so you fall forward and then, more of the same . .
So, what helps, along with all the points the others made, is to really think of riding the HIND end of the horse. I literally imagina myself riding a horse that has only two legs, like an Emu with a horse neck and head; I sit back on top of that "emu"s feathered body, right over those two legs, and I kind of squeeze up and in front of me. Funny image, but try thinking about riding ONLY your horse's hind legs, just for that moment of gathering him and sending him into the canter. And be sure to relax the instant he taeks the canter and go with the flow.