First I'll address siting the canter. Most riders have difficulty in sitting the canter or any gait when some part of their position is out of alignment. Remember that the basic balanced position requires a straight line from ear to shoulder to hip to heel. Without a picture of your riding it's hard to tell where you might be out of line, but do a self check on yourself. Are your legs in front of you, making you sit on your seat bones in a chair seat? If so, bring your lower leg back, relax the front of your calves on the horse and let your leg down long.
Many riders can't sit the canter because they perch and their weight is on their crotch and their thighs. This makes the horse stiff because any energy generated from the hind end that should flow to the bit is getting stuck at the withers where the rider is pinching. If you think that's your problem, work on bringing your seat under you more (think tuck your butt) so that there is equal weight between your pubic bone and your seat bones. Relax your thighs. Your calves are the leg muscles that talk to your horse, never your thighs. Your thighs will pinch the back and disallow the horse to relax and round the back. This will relax the arch in your lower back and allow your seat to follow the horse's energy and the horse will be able to move through his back to the bit.
As for bending, never never never NEVER pull the inside rein back. Pulling back discourages the inside hind foot to step under, which is what you want. You create bend through what we call "Balancing Aids." Your balancing aids are your inside hand, inside leg and outside hand. The inside leg will first ask the horse's barrel to move to the outside and for the back to bend. The inside hand asks for flexion by bringing the rein out (to the side) not back. To catch the horse from falling on the outside shoulder and drifting out, the outside rein half halts. Your seat must follow the curve that you wish the horse to create. Many riders take this to mean that they should pull their inside seat bone back and their outside forward, however, this will only stop the motion of the inside hind and send the horse instead on a straight line with the horse crooked. Both hips need to follow the motion-- the inside hip follows with the inside hind, and the outside hip follows with the outside hind, but travels further than the inside hip. Both sets of the horse's legs are making a circle, but the outside leg is making a slightly wider one.
Hope that helps. Have fun at camp.
