I think it would be really beneficial for you to have a flat lesson with an instructor to work on your seat, esp at the canter. It's really stiff and looks uncomfortable for a horses back. Work on relaxing your lower back a lot and opening/ relaxing the hips to encourage them to move with the actual rolling movement of the canter instead of hitting the back and bouncing off the motion.
I've found yoga stretches to be very helpful for these exercises.
We are not human beings having a spiritual experience. We are spiritual beings having a human experience. -Pierre Teilhard de Chardin