I think you should try to slow down your shot a little. Try to reach a position at full draw where you don't feel stressed, where you feel like you could hold all day if you wanted to. This would be a position where you feel like you're holding with your bones rather than your muscles. Although your back muscles will be involved, they are large muscles and usually don't feel stressed when you hold them for a while. I notice in some of the shots that your bow arm/hand is moving around quite a bit after the shot. At first, I thought about advising you to work on your follow-through. But after several viewings, I think you are following through okay, and the movement I see is just a natural reaction of your body to unnatural forces during the shot. Experiment around with slight changes in your alignment until you eliminate as much of this post-shot movement as possible. Do you think you could be torqueing the string or your grip, and this could possibly be causing the post-shot movements? Try raising your elbow some, as Arne suggests, and see if that helps.