You're absolutely right and the parallel about musicians is a valid one. Mozart was an anomaly. He composed entirely in his head and by the time he got around to writing down his work, it was like taking dictation. Every note was perfected before it was committed to paper in his brilliant mind.

However, Beethoven, who was no less a genius, began his work with embarrassingly awful themes which he painstakingly reworked (and he was kind enough to leave reams of notebooks with his scratchings that detailed his process) into the glorious music we know him for today.

Writers have to be strong enough to be willing to destroy their work without pity. That's the only way the best of what we have to say will be heard.