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Sounds like your missing some internal rotation (IR) of that shoulder. Having the hand behind the back and having the shoulder dump forward is a good assessment that your lacking IR. Check out the hang archetype on the getting started page:
I would focus on some IR mobilizations. There are plenty of videos addressing the hang archetype and IR deficits. Here is a good place to start:
Cleaning up the hang archetype and resorting your IR will also help restore full capacity in that shoulder, reduce compensations, and should help you handle more volume in your workouts.