I used to have an issue with sitting the canter as well. I didn't even fall off, I was just scared! I got over it by having my trainer put my horse on a lunge line and me hanging on to the "oh crap strap" on the saddle (do you have one of those?). I held the strap until I felt comfortable enough to not use it. Then I held the reins instead. And finally I got off the lunge line and now LOVE cantering.
I hope that helped!