Dave,
Your drawing hand appears tense and your elbow is not back all the way. You are using your arms and not getting on the back muscles. Try this to get a feel for the back.
Take your bow arm and put an big bend in it, as if you were going to shorten your draw by two inches. Now draw back as far as you can, pushing with your bow arm just enough into the bow to maintain the bend. Keep pulling with the intent of reaching full draw even though your bow arm is bent. When your elbow is back as far as it can go and you are locked in to anchor, keep it locked in by pulling as you push your bow arm forward slowly. No doubt your bow arm will be shaking but as you push forward and pull back, the shake will fade and there will be a point of absolute steadiness. That is the point where your muscles and bones are lined up and you are both pushing and pulling equally.
Hopefully this came across right. It's kinda difficult to explain in writing. If you take it slow and get a feeling for good back tension, the timing will work out and everything will flow together.