Do you have access to a horse with a lot of movement? Like wamblood, big, bouncy, couldn't-sit-if-my-life-depended-on-it movement? Both the horses in these videos have small, flat movement, with little suspension. Riding a big moving horse might help you focus on balancing, and absorbing motion, rather than just sitting there and be taken for a ride. Sometimes it's hard to feel the difference on small moving horses since there isn't much to absorb in the first place. Just a thought, riding a big moving horse really helped me get my sitting trot under control and I had to learn how to really ride it properly.