I suffer from this… at least every time I start a new job I do, it is terrifying:

Research that began in 1978 with the work of psychotherapists Pauline Clance and Suzanne Imes found that many women with notable achievements also had high levels of self-doubt. This deep lack of confidence—which couldn’t be equated with anxiety or other disorders—appeared to involve a deep sense of inauthenticity and an inability to internalize their successes.

These individuals often have the belief they are “fooling” other people, “faking it” or getting by because they have the right contacts or are just plain “lucky.” Many hold a belief they’ll be exposed as frauds or fakes. Impostor Syndrome goes far beyond normal bouts of self-doubt.

Referring to impostor feelings among career women, public speaker and consultant Valerie Young, Ph.D., notes that their fears can “prevent them from fully enjoying their success and seizing opportunities, and can cause them to overwork to compensate for supposed deficiencies. 

- http://www.forbes.com/2010/02/22/imposter-syndrome-professional-fraud-forbes-woman-leadership-psychology.html