I have some idea of what you must be feeling, but don't worry. My DD 13 is on her first week Rehab for both shoulders, which have got progressively worse since it all kicked off big time in January. She has been suffering from continual spasms (jumping in and out like a nervous tick) in her shoulders for the past couple of months with sublux between 2 a day to metronome speed.
Physio and OT have been wonderful trying out lots of different things to try and find out what makes her shoulders tick 'literally' like a clockwork. They are very honest and say they're not there for a miracle cure she won't be fixed by the end of the 2 weeks. But like you there is a lack of muscle control at the back. If you're prepared to follow what they suggest and do the many small exercises they give you, you can feel a sense of achievement and feel like you are gaining some control back in your life.
I hope that helps a little, knowing my ramblings probably not
Let me know if you want to hear any more.
Whenever you go take some food with you, it is dire.
In the meantime you could try acupuncture really helped give s her a break from spasms from 1 to hours usually depending on the time of the month.
Single Mum to DD finally confirmed EDSIII and Irlens syndrome