"But how much can adults learn if they are constantly comparing themselves negatively to others (children, no less) and feeling guilty about every unmusical wrong note?"
So the paragraph that you quoted above just appears to me to be much ado about nothing. The basic premise that the author is asserting here doesn't even apply to me.
Let's contrast music study with golf, another favorite self-improvement undertaking of mature individuals. Golf is challenging and unpredicatable, just like music performance. Happily, though, golf isn't played by precocious kids; therefore, no need for self-deprecating comparisons to little prodigies. Ezra