Hello Sarah!
Shoulder problems are usually linked to your seat position, it is probably too much backwards.
You need to develop your back and abs muscles (swimming is very good for that, it stretch all your body and is very good).
Try to be focus on staying straight even when you're not on a horse (while seated, walking...) and on a horse, use a whip in your back behind your elbows to keep a good shoulder/back position.
I hope it'll help you, good luck :)
