“Sleeping on my arm?”

I've had the same job for 16 years it's physically demanding and gets worse every Xmas time. My right shoulder is in pain makes it hard to sleep at night. I sleep on my side as well. The pain extends down my arm and my index finger feels asleep. Oddly enough when I sleep on my left side keeping my right arm straight down and with a pillow between the legs it pains even worse and keeps me up all night. If I sleep on the side that pains. Then there is comfort it is like it is being compressed to pillows and it feels good and I can sleep. I know the ideal thing to do is sleep on my back I must train myself to do this but it is hard. I'm scared sleeping on the painful side will make it worse. Yet it's the only way I can sleep in comfort What can I do? This has been ongoing for many years.