How To Treat Sharp Pain And Discomfort In Shoulder Caused Due To An Injury?

My right shoulder and top right half of back was beaten severely many years ago, there was periods where i had skin sensational difference in a small part of that area and after time the area has increased to almost the whole of that area. with deep sharp winded like pain and discomfort intermitant over the years. The pain then went into the top of my right arm, like a band tied around inside of arm, there is also i feel a difference to my right breast drooping, due to what i feel is no support or muscle control in holding that side of me up and i can feel this under my right arm and all round to the back part of that sholder... i have limited movement in my arm (can t lift it high or move it back, or lay slanted on my back as this also creates severe pain and discomfort)and an almost constant nerve like pain which heightens with the wrong movement . There is also pain when non-movement. Mentioned the initial pain the time it happened and was told it was muscular, so didnt mention it again until now (20+ yrs) my qyestion is what could have happened to my shoulder to cause this pain/damage there is now and is there anything i can do to fix it or make it better.? Thank you for your time.