The most effective exercise that I've found are these 'wall angels' :
I do them slightly different in that I start at the quarter to three position and hold the arms there for 5 seconds or so. I then move up to the ten to two position, five to one and finally the twelve 'O' clock. I really try to reach/stretch out with the hands. I just do one or two, perhaps five times spread through the day. After doing these my shoulders always feel a lot lighter.
I've suffered huge amounts of upper back pain over many years, with nasty muscle knots. That led to numbness in the arms/hands. Not playing and months of rest from the Guitar was the only way of alleviating the pain. Doing the wall angels, over a period of 3 months or so, seems to have altered my posture whilst playing. I'm pretty sure my shoulders were closing in on the Guitar, with the right arm bearing down too heavily on the lower bout. Now my chest feels much more open, the right arm free and the shoulders in a much more neutral position.
Try them. If you find the exercise difficult, ease your way into them. Each day it should get a touch easier. Make sure everything is pinned back against the wall, including your head.