Impostor phenomenon.

Impostor phenomenon, explained simply, is the experience of feeling like a fraud (or impostor) while participating in communities of highly skilled participants even when you are of a level of competence to match those around you. Impostor phenomenon is highly correlated with individuals who are successful; arguably success here means success as perceived by others.
— Angelina Fabbro

Taken from a great piece by Angelina Fabbro, on one of my new favourite blogs: The Pastry Box Project. The Pastry Box Project being defined as: 

'30 People Shaping The Web. One Thought Every Day. All Year Round. Sugar For The Mind.'