Stubbornness.
I'm not two rivers mule-headed, but I'm far from wishy washy.
I think that I'd have a snapping disease like Preservation, but you have to go around having a normal day even though I'm cursing you
My magic would be musical, and what you have to do is convince me using the music to grant you the power. I'm not thinking incantations or anything, I just love music so what you have to do is use music to stop change the stubbornness.