Most people think of their performance curve as a shark fin. They believe they do excellent work most of the time, average work sometimes and substandard work very occasionally.

However, this is a classic case of illusory superiority bias in action (it's the same reason 75% of people rate themselves as above average drivers).

The reality is: your performance curve is much more likely to resemble a bell curve than a shark fin.

But, this doesn't mean you can't improve. You can. Through deliberate practice (challenging yourself and gathering feedback), you can move the bell curve to the right. You're essentially changing the frame of reference that you use to rate yourself. By trying to make sure each subsequent performance is better than the last performance, you can slowly move the bell curve.

For example, you may think you're a great basketball player. But, compared to the average player in the NBA you're terrible. On their worst day they're still exponentially better than you are on your best day. The same holds for golf, marketing strategy or design.

You have to compare yourself to people that are better than you. This requires a humility that some people can't stomach. It's hard to admit that you're not as good as you think you are (or as good as you pretend to be). But, that's the only way you can move the bell curve to the right.