Try sitting the canter without hands- aka, holding your arms out to the side, putting them on the top of your head, etc. This should help you get used to the movement of the canter and strengthen up your leg muscles for sitting at the canter.
If you're holding on to the pommel you may still need more support to sit the canter correctly and not flop all over the place (not saying you do but you get my point xD).
It takes a while to get the motion of any gait down. I'd need to see a picture or video of you riding to make even a slightly accurate guess. If you don't do very well in your next group lesson at the canter, don't feel bad and don't be afraid to revisit the lunge line if you have to.