I like to stand in the saddle, flex heels down, bend the knees and then sit. Your knees appear locked and not helping keep with the horses motion. Your feet should be under your center of gravity so you can stand in the saddle if you need to. Try it and see if that helps. Also use your thighs to grip lightly and your calves to let the horse know how fast and which direction to move. Nearly all signals are the horse moving away from prssure. You just have to figue out where and how much pressure to use.
The only thing I noticed is that you are madly bracing against those stirrups! Relax your legs :) Those mickeymouse ears on the Aussie saddle will keep you from flying forward.. and your seat will keep you in the saddle!