I am sorry that you are in pain. I often hear of people who have pain in this area of the body, and I understand that it can be very uncomfortable.
To ‘diagnose’ yourself through online sources is really not a good idea. You write: “I know it’s a sprain.” Are you so sure? I never heard of a sprain of the upper trapezius (does not mean it does not exist). But I have heard often of neck problems because of the spine of the neck, or of something wrong with the shoulders. It may be a problem with your posture. And who knows how many other reasons there can be.
Where I mean to say with that is that there is no way for any of us to make any recommendations of what you should do. We are fitness professionals and happy to to work with your doctor and/or physical therapist if the problem, after a proper diagnosis, falls into the area of our expertise. You need to take this step, though, to make sure that your problem gets the right solution. My answer has nothing to do with the fact that you are only 13 years old. It would have been the same for anybody else.
I wish you good luck and hope that you will soon be better.
I would talk to the doctor.
Mac Dodds M.A., CSCS
Live Your Best Years Now