Hey Kellie,

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:
(https://thereadystate.com/getting-started/)

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:
(https://thereadystate.com/pre-movement/hang-archetype/)

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.

-Mike