Ear trombone player with left sided shoulder pain. I also play the trombone. I am also lefthanded. I am one of the rare people who play their trombone lefthanded. My band director was a clarinet player, and he didn't notice that I put the trombone together so that I hold it with my right hand and move the slide with my left hand. now that I've done this for 60 years, I don't think I could learn the other way. More about me before I get to you. At the age of 72, I've developed arthritis in my right thumb. I don't think it's from the trombone, but it certainly makes holding the trombone up very difficult. because of this, I wear an orthopedic thumb brace that is useful because it holds the thumb and a fixed position. However, I have also looked on the Internet and found several thumb braces for trombones. The problem for me is that they only work if you hold the trombone right handed. I cannot use them, because they don't work in a mirror image. There's only one, a strap from Yamaha, which is symmetric and so I can use it. I don't think it works, as well as the rigid braces that clamp onto the trombone would be. now to you. Take a good look in the mirror and standing still. Are your shoulders at equal height? If you have a curvature of the spine or neck, which is called scoliosis, this could be causing your problem. You don't have to be a trombone player to have this condition. it is relatively common, maybe 5 of the population. Females have it more than males. The best way to tell is to bend over and have somebody palpate your spine and see if it seems straight. If this is the case, then you could talk to your family Doctor who could order some basic spine xrays not CTscans, and if scoliosis is found, then refer you to a spine doctor. At your age, correction could be a good thing. If you wait until you're more mature, the improvement that you might get may be a lot less.