The main thing that sticks out to me is how your lower back is stiff and hollowed out. It looks like you're almost forcing yourself to sit in a perching position, so you're tightening up every muscle in your back. Your lower back should be loose and absorbing the motions... not slouching, just supple and relaxed.
Another problem I see, which really comes out when you canter, is that pinch in your knee. Do you see how your upper body is tipped forward, and your leg has slipped back? Try to lift your hands up, don't bury them in the mane, and rock
back, don't lean forward. Keep your body in a straight line from your ear to your hip to your heel. To help you do that, you need to sink more weight down into your heel and get the knee off that saddle! I think once you tip your upper body back and grip with your entire lower leg, not just your knee, you'll find it easier to use all of your seat, you'll be more relaxed and comfortable at the canter, and you won't have to crack that back so much.
