Thank you for showing the short video. It is so much easier to make a helpful critique when there is a video to see. And I applaud your courage to post it.
There are s me good comments above. Mainly, I agree that you may need to go back to trot, sitting trot, walk without stirrups. If you are really solid in these, then cantering will be much easier.
I think teachers often advance their students too rapidly and this is careless of the horse's and riders' back. You can either spend more time on the trot/walk and then when you get to canter, be more solid (thus less trauma for backs) or spend more time whacking around on your backs until you finally put it together at the canter. Time is required, either way.
There was a moment in the video, at #28 second, when he started his second canter, you were WITH his motion for two steps or so. Look at that moment. you were moving up when he came up and down when he went down, and then as he got a few steps further, your rythm and his parted ways. Then you were constantly behind his rythm. In actuality, your position in the saddle is not bad. Really. once you are able to be relaxed and loose enough in the hips to allow them to follow the horse 's movement, your rythm will sync with his.
Sit in middel of saddle, up nice and tall. Lift your thigh off the saddle , roll the fleshy back part of the thigh away from the saddle (use your hand to pull each one out, then in this position, put your thigh back on the saddle. Your knee will be more directly forward and you'll have more leg cohesive to the saddle. Do NOT tuck your tail bone under . Sit nice and square on your two seat bones.
Walk along and visualize your seatbones as two prongs of an electrical cord/plug. Plug into your saddle and think of yourself as riding INSIDE your horse, about 6 inches under the saddle . Your pelvis belongs to him and must go with him. If you are tight in the hip or abdomin, this will not be possible.
Breath, laugh sing. Trust your hrose to carry you. He does this all day and is very good at it. All you need to do is LET him carry you.
Thoughts for you to have.
You will get it, don't worry . We've all been there.