Choose a purpose that satisfies yourself at the moment. Stick with it for awhile. If you try that purpose for awhile and it doesn't work, choose a new one. It may take a lot of experimentation with various other "purposes" before you realize your "true purpose."
If you truly desire to be a great musician, that is great.....that isn't really a purpose though... What is your purpose? To bless the world with your musical creations? AND...just because you want to be a great musician doesn't mean that your whole life's purpose has to revolve around that....
(BTW>>>I think you should pursue your musical desire and see how it goes...)
Good luck on your journey!