Alright, here's your problem: your center of balance is off. At the beginning of the video, you tipped forwards, and after a few seconds your leg slid forward and you tipped back. Your center of balance is always changing. This is because your legs are unstable. When your legs are unstable, your upper body must do all the work. For every pound that moves forwards, a pound must move back. Your core for posting is all in your legs, and like bsms said, the only way for you to post is to move your hips over your legs. But if your legs are too far forwards, your shoulders must come back, and you must rise higher in order to stay balanced.
The solution is to strengthen your legs and work on keeping them in place. Posting without stirrups is a great excercise. Don't think about posting up, down
, instead think about posting forward, back
. Keep your heels waaaayyy down, and never ever grip with knees. Strengthen your leg muscles. Squats lunges, stretching your heels, touching your toes... those are all good leg excercises.
Your stirrups may also be too short. When you hang your leg down, they should be at your heel and no shorter. For flatwork, you can even lower them one or two hole and then bring them up again while jumping. Just try to stretch your legs down and put all the weight there.
I hope I was helpful. Good luck. :)