What is happening is that you are pushing the ball a little bit too far to the right before dropping the ball into your backswing. There is about a 2 inch gap between your ball and where it passes by your body. Assuming that you are pushing the ball out towards your target line, try standing 2 boards to the right of where you normally stand. This should bring your armswing in closer to your body and straighten it out so it doesn't go behind your back.

I mentioned the towel-under-the-armpit exercise before going back and seeing that CoachJim had already discussed it.