I used to fear cantering too! But I got over it by only cantering in smaller, more enclosed arenas, such as round lunging ones. Once I was comfortable in there and felt confident that I had control over my horse during the canter, then I moved to the large arena. Hope this helps! Xx
As odd as this sounds, the best way to stay still when cantering is to allow yourself to move with the horse (but not actively moving yourself). Allow your ankles to absorb the movement.
The stiller you try to keep yourself, the more you're going to bounce. Relaxed but strong, using your core (stomach and back) muscles to keep yourself upright.
And don't forgot to breathe!