Hi my horse has a pretty stiff shoulder and im not sure if that's something that should be taken care of by a vet or if I can fix it on my own?
You have to figure out what could of caused the stiff shoulder.
Poor saddle fit?
Unbalanced rider - shoulder is stiff to compensate?
Back could be out?
Etc, etc - your horse could of rolled or over played out in pasture?
I would suggest getting a Chiro out to adjust your horse, and I would assess your saddles fit. Can you get a Professional Saddle Maker/Fitter out to ensure that the cause of your horses stiffness and pain isn't from your saddle?