When you next have a chance to canter the horse, try this:
REally think about letting the horse CARRY you. I mean don't try to do the work for him. Think about trusting him to do his job and you just ride ON him. As he is going along let your legs just drape down on him as if you have no control over them. Feel how he lifts up your right hip as he canters on a right lead. Sit up tall, and LET him carry you. All you have to do is sit up tall and NOT do any work. Let him do it, and stay out of his way by not firing any of your muscles except the abdominal/back muscles needed to stay sitting up straight.