If you use the blank bale, instead of doing the swing draw, hold the bow out in front of you and squeeze your back rhomboid muscles, to understand the feeling, grab your hands in front of you and pull, you should feel it in your back, if you feel in only in the shoulders you are doing it wrong. By getting your back involved early in the draw, it is easier to maintain that throughout the shot sequence.
As far as a drop down in weight, you can make a pvc bow, lots of how to's on youtube. It will cost next to nothing and give you a great warmup bow. It's important to know how a shot sequence is supposed to feel like. You can do this by blind bale or using a light weight bow.
But if you can't handle the bow weight, your form will never develop.