I've recovered from various long-term muscle injuries myself using trigger-point massage in the affected and related areas causing the pain, sports massage to break down muscle scaring, stretching, and epsom salt baths before bed (high in magnesium which regulates calcium/magnesium imbalances often causing cramps and aches in various parts of the body - often the magnesium is deficient). Surprisingly, cold showers also help with muscle recovery, sleep, waking up, healing, boosting the immune system - but never take them before exercise, stretching, or massage.
Trigger Point Massage is a "good pain" type of massage that targets complex muscle knots often the size of a pea and often located separate from the region of pain yet exacerbating it when massage. These are caused by injuries or repetitive stress injuries through various actives with poor posture of excessive unhealthy effort without proper form or recovery. A book called The Trigger Point Therapy Workbook by Claire Davies explains this further. I recommend getting a decent massage cane, double lacrosse ball, and foam roller. These can all aid in proper self massage of the shoulders, back, and neck which all could be contributing to the shoulder pain. Massage activity should utilize gravity by laying on the foam roller or lacrosse balls and moving slow and deep on and around the affected areas - getting the neck, shoulders, back, and in between the shoulder blades with the lacrosse balls. Then massage the deeper hard to reach areas of the back and shoulders with the massage cane while sitting and relaxed. You can also massage with the lacrosse balls up against a wall (sometimes works a lot better than the ground), go as deep as you can handle while the pain is solid in the effort but not unbearable and ridiculous. Focus particularly on any areas referencing exasperated pain in between your shoulder blades. A dozen or several dozen repetitions should suffice each day.
As for Sports Massage, these same tools above can be used, however, unlike Trigger Point Massage, this technique is focused on breaking down stiff crusty muscle scaring under the skin that definitely required an injury to form. It's also a good pain type of massage that provides relief and greater flexibility if muscle scaring is indeed the case. The pain will be exacerbated by massaging the exact area of pain, and you will feel pretty intense stiffness. As for the massage technique, it is similar to the trigger point therapy above but focus much more on the actual area of pain and loosening up stiff muscle tissue. The pain may actually be higher in this type but you should be getting relief each day if done right. A dozen or several dozen deep slow massages should work too. From my experience, scar tissue massage takes more effort than trigger point massage - the stiffness is higher and the pain is a bit more intense but the relief is real and I went from not being able to run for months to now being able to run pain free for almost a decade after resolving a little scar tissue that would pinch every-time I ran - months of not recovering after a fall.
NOTE: any self applied massage is done at your own risk. If you are truly unsure of your injury, seek a medical evaluation however I recommended staying away from painkillers unless the pain is absolutely unbearable because they are only masking the issue, not solving it. Most cases of non serious injuries and mere knots or scaring can be solved with these massage techniques - relief should happen slowly over a period over a week or month. Some cases may even find relief the same day which is encouraging to continue as long as necessary to recover - which may still be weeks to further prevent the pain from coming back.
As for stretching, practicing flexibility focused yoga courses will improve your flexibility tremendously. A lot of great apps exist out there, but I use Yoga Studio because you can customize your own workout videos after you discover which poses help you most. Also practice slow breathing and Proprioceptive Neuromuscular Facilitation (PNF) stretching which is essentially applying some muscle squeezing for a few seconds into your stretch, then releasing further. Breathing for at-least 4 seconds on the inhale and exhale will boost flexibility, reduce stress, and speed up results over time. Pain should be a moderate balance between effort and relaxation - low/medium pain, not high but also felt effort. PNF stretching helped me release my own stiff upper back and shoulders, now I can actually clasp my hands behind my upper-back both ways (one arm tossed behind its shoulder, the other behind the hip, fingers hooked together, shoulders stretched). PNF allows those stretches that even tons of stretching alone wouldn't allow. In a sense, this technique also breaks down knots in the muscles, massaging them for a few seconds mid-stretch. Yoga not only increases your performance in many exercises but provides a pretty significant calm self control/higher tolerance for stress effect over time that is long lasting.
