What should be the mindset of a "black belt" musician?
1) They worry less about the opinions of others in practice. The black belt is the highest level of martial arts recognition, because learning has become a lifestyle for these people. A bad day or difficulty learning a piece does not worry them, because they have replaced the self-centered drive to prove themselves with a curiosity about music. Now they focus less on end goals. The journey has become their teacher
2) They know how to avoid unnecessarily injuring themselves. Practice happens at a safe speed, not sight-reading and cram-learning music. When you throw things together, you are paying less attention to your technique and, consequently, risk injury. A black belt musician also tries to improve every area of his life, even outside of music. This person knows that those "outside of music" things effect the way one practices and performs. An unhealthy lifestyle or relationship restricts your musical capabilities.
3) They are "scientists" of music. A black belt musician doesn't torture themselves with over-long hours of practicing. They don't rush learning. The temptation is there to try to find short-cuts, but there really is no such thing as a short-cut! A black belt musician perfects what they know by drilling and trying to discover new ways to apply what they know and more efficient ways to practice. This makes them more effective. You cannot practice 7 hours your entire life. By being a scientist of music, you become more effective at practicing and accomplish just as much in less time.
4) They know that music is supposed to be fun and brings a lot of benefits. Music should never be a chore or a job. If you mentally treat it more like a big hobby, you think of improving your skills ALONG WITH thinking of new ways to make music more enjoyable for yourself and your audience. That enjoyment keeps you from getting burnt out. All people tend to accomplish more when they are secure and excited! Have reasonable expectations of yourself, and stop to smell the roses along your journey with music.