I do see improvement. Defininte improvement over the last few vids you posted.! Bee is such a sweet horse.
I am not sure how to describe specifically what to do, but somewhere in your body you are not absorbing the horse's movement, so you get tossed around a bit, like a doll would if you tied it onto the saddle. Not THAT bad, though. I wish I could see things better. IT's so far away, it;s hard to tell what is going on. I do see air behind your knees , so they must be coming off the saddle. Perhaps you are gripping up with the lower leg? What does your insructor way?
Its not really a chair seat, rather an overall lack of stability down through the legs. But, I definitely see improvement!