Get yourself referred to a shoulder consultant and a MRI scan of the whole shoulder.
I haven't been able to sleep on my left shoulder for 3 years, even after a shoulder op - I know what you are going through. Invest in a few pillows and wedge one in your back so you can't turn over, and one on your front to support the damaged shoulder - i.e. raise it up off the bed surface.
You will most likely need an a/c or rotator cuff op or a decompression depending upon what they find.
Rotator cuff they fix up any torn ligaments etc. etc.
A/C joint, they take off 5mm or so of bone off the end of the clavicle so it frees up the A/C
Decompression - shave bone off the various parts of the shoulder so it frees up muscle and ligament movement.
PS none are 100%. Mine was 80%. Mechanically it's now good, but I'm still in loads of pain, 9 months after the op. (that's not counting nerve damage).
Certainly back to GP and get to a shoulder consultant.
Let the solicitor at least get the liability sorted. Think about seeing a physio, as they will give you a very good idea of what's going on in there, and if liability agreed, get some treatment paid for as an interim. It's taken me 2 years to get my £2k of physio fully paid out. Now going back to physio out of my own pocket, until final claim gets sorted.
It's going to be a long battle I'm afraid, so speak to the solicitor and plan any treatment, courses of action with them.
PS feel free to PM if you need to.