Hi there. It must be very hard to try and learn solely from reading and watching videos. I would be really fun for you to take some lessons. I think you'd learn in one lesson twice what you can learn in a week of watching videos.
Ok, so for the critique. Your legs are much too far foward, in a chair seat. The stirrup is too long. you need to shorten the stirrup one or maybe even two nothches. yeah, two. get your feet back under you, not if front. If you horse were to magically disappear from under you , you want to be positioned such that you will land softly on your own two feet. here you would land on your tailbone!
This kind of position makes it hard for you to be in motion with your horse as it posts. you will always be "behind" the motion and will end up pulling on the reins to keep your balance.
try riding this little horse around a small arena or paddock with almost no rein (ride on the buckle) at a walk at first. get yourself lined up and try to rise up and down and have no help with the reins. just your real balance. you will need those feet under you, not out in front.
Your horse is throwing up its' head and hollwoing out its back due to your way of posting might be bringing you down too hard on his back. Or, it could be the saddle fit is not good , or a combo of both.
can you post pics of the saddle on the horse where you normally put it, no pad? side , angled, and front /back views.