Many a high achiever worries they will be exposed as a fraud. They actually feel like they're phonies. There's even a psychological name for this worry - it's called "impostor syndrome." The clinical definition is a “psychological phenomenon in which people are unable to internalize their accomplishments.” According to University of Pennsylvania psychologist Dr. Joan Harvey, "victims of the impostor phenomenon persist in believing they are less qualified than their peers, and suffer from the fear of being found out."
The good news is the feelings are normal when say starting a new job, but should fade over time. Those with the syndrome, however, only seem to feel more like impostors as their accomplishments grow. "Instead, because each success is experienced as either a fluke, or as the result of Herculean efforts, a pattern of self-doubt, rather than self-confidence develops" per Dr. Harvey.
Fortunately, often simply talking openly about the feeling with those that feel the same can help those with the fear. And if it's any consolation, even Richard Burton thought he was an impostor.