Beyond choosing the right mattress and pillow, there are other things you can do to improve your shoulder pain and sleep (and feel) better. These include:
Avoid sleeping on your stomach or on your bad shoulder. Either of these sleep positions can exacerbate shoulder pain. Instead, try sleeping on your back or side (with your bad shoulder facing up).
Perform daily stretching exercises. Gentle stretches of the shoulder, triceps, and chest can strengthen your muscles and relieve shoulder pain. Try these exercises recommended by Harvard University.
Pay attention to your posture. Avoid slouching when you sit or walk. Use pillows in your chair to help you sit up straight, and walk with your shoulders back and your neck straight.