From what you say, it seems like you ARE gripping with your lower leg/calf, and you never should grip the horse at all.
The canter is a diagonal gait, so your seat should be slightly diagonal while sitting the canter. Whichever lead you're on is the hip you want slightly forward. Your inside leg should be at the girth, but not gripping. It's just there, and your outside leg slightly behind the girth, again it's just there.
I would suggest some core exercises, and strengthen your lower body as well. Your balance should be centered in your seat and anchored by your legs. When you grip with your legs, you become top-heavy, using your legs as a fulcrum and you flop around in the saddle. You should flow into and from your seat. Try to remain upright. Learning the canter takes time, just like the other gaits.
Good luck :)
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