I saw the vid, but I don't know how MIeventer can tell about the saddle. I feel like I'm peeking out and can hardly get any detail.
You do post too "loud", but I think that is because you are posting off your knee. You are levering yourself up and working a lot harder than necessary. When you can get you leg more under you and post with your weight comeing all the way down into your stirrup, it will be much lighter, both goin up and coming down.
There was some discussin about the upper body coming forward or backward, according to whether or not the lower leg was forward or back.
Here's my experience: If the lower leg is too far forward, the upper body will have to compensate to keep the body balanced. The only way to not do that is to resist by pushing hard into the stirrup, but when posting , if the lowe leg is too far forward, you HAVE to more your upper body also forward.
If your saddle is not sitting correctlly on the back of this horse, for reasons of poor saddle fit or because the hrose is by nature very "uphill' in build, the front of the saddle will be too high. You will end up kind of sitting on a "hill" and will constantly be struggling to "climb" it every time you lever yourself out of the saddle. This also results in the rider often falling behind the motion (the trot rythm) of the horse, and then landing not as soft as they should in each down stroke of the post.
Look at the saddle sitting on the horse's back. See if the back (the cantle) is about 1 inch or so HIGHER than the front. If the front is higher than the back , then the saddle's position is off. Might indicate that it's too narrow in front, too.
How tall are you? What size is that saddle?