You just can't shake that insidious little voice that whispers "You're a fraud". It rattles your confidence, undermines your pleasure in your achievements and dogs your working day, leaving you secretly convinced that your boss and colleagues will eventually realise you're just not up to the job.
We've all been there; whether it's the offer of a promotion we secretly fear will expose our inadequacies, or a plum assignment that we suspect we won't carry off, most of us are familiar with the feeling that we just don't have what it takes.
It's Impostor Syndrome, a condition that leaves high achievers feeling that, despite the accolades, they're not what it says on the tin.
And although Impostor Syndrome affects around 70% of us, women are more susceptible to it than men -- and they're more interested in finding a solution, according to US psychologist and expert on the condition Valerie Young, who is about to publish The Secret Thoughts of Successful Women: Why Capable People Suffer from the Impostor Syndrome and How to Thrive in Spite of It.
That's because women are operating in a world that presumes they're less competent to begin with, says Young.
"Women are also more susceptible because they're more likely to internalise failure and blame themselves whereas boys and men are more likely to attribute failure to something outside themselves. Women are the ones who are raising their hands looking for help with it -- and this is why I decided to write for women specifically," she says.
The good news, though, is those of us who suffer from it are in excellent company.
Michelle Pfeiffer has said she fears people will discover that she's really not talented and that her successes have been based on a sham. While Oscar-winning actresses such as Renée Zellweger, Kate Winslet and Jodie Foster have gone on record with their fear that they're fakes.
Movie star Mike Myers has famously quipped that he's afraid the 'no talent' police will show up one of these days and arrest him.
People in all walks of life are dogged by the belief that some day they're going to be 'found out.'
Whether we're students, athletes, salespeople, doctors or jet-setting entrepreneurs, many of us find it impossible to acknowledge the talent, intelligence or creativity that brought us success.
No amount of positive feedback, praise or offers of promotion will convince us that our abilities are praiseworthy.
Instead we put it down to luck, hard work, personality, being able to fool others into thinking we're smarter than we are -- or simply being in the right place at the right time.
It steals your peace of mind, your self-confidence and your happiness -- and it can hinder your career, says Young.
The more successful you become, she says, the more likely you are to have experienced it at some stage:
If it's not tackled, it can even deter people from going to college. In the workplace it can discourage some sufferers from chasing promotion or challenging assignments.
Others never enjoy the satisfaction of a job well done because they feel they'll be found out 'next time'.
"There's no joy, only relief when they succeed. It's a stressful way to live, always looking over your shoulder and waiting for someone to tap it and say you shouldn't be here," says the psychologist, whose book hits the shops next October.
"It's like a tape that keeps playing, looping over in your head," she explains, adding that it can unsettle some people so much they won't even raise their hand in meetings.
"These are ways that you go about making sure people don't find out that you are a 'fake'. It's an unconscious strategy to deal with the stress caused by Impostor Syndrome."
Some people deal with it by over-working, says Young -- and then when they're successful they attribute it to the fact that they worked harder than everyone else:
"It becomes self-perpetuating -- you're afraid to let go. The harder you work, of course, the better chance you will have of being successful, but it's still an empty feeling."
It's a common problem, admits Brian Colbert, a Dublin psychotherapist. "I work with successful actors and sportspeople and business people -- household names -- who feel they're not good enough to do what they're doing and have this sensation that they'll be found out," he says.
One of his clients, a man he describes as "a jet-setting entrepreneur and millionaire", for years failed to find satisfaction in his achievements because he never felt they resulted from his considerable business skill.
Instead he put his success down to hard work and luck -- and was always waiting to be unmasked by people he considered better qualified than him.
Impostor Syndrome crops up regularly in the higher levels of the corporate world, says Business Coach Finbarr Sheehy.
"In a lot of companies there's an atmosphere of macho posturing. People appear very confident. There's a fear that if they don't appear like that they will be perceived as not performing," he says.
Beneath their outward assurance however, such people often experience secret but debilitating self-doubts.
"They'd confide in me that they doubted their capacity to do something , that they feared they were not effective. There's this void inside them.
"In some cases the higher you go in an organisation, the more cases there are of it. The people that I was dealing with would have been at quite high levels of the corporate world, on salaries of up to €200,000.
"Typically it would involve people in very senior positions, whose role would be critical to the company. Sometimes they'd tell you that they fear they're not capable of it, and wonder why they were picked for this job.
"What I do is focus on a person's achievement and qualifications, and demonstrate that their achievement is not just down to luck or hard work."
At the heart of the problem is how a person defines competence and how they handle mistakes or failure, says Young. If you insist on doing everything perfectly all the time, you're setting yourself up for problems, she warns.
Colbert, author of The Happiness Habit, agrees. In the minds of some of his clients, he says, they will never be good enough: "They decide that only they know what 'good enough' will feel like -- when they are good enough."
Such people are searching for the feeling of achievement he says, but they often don't feel it because of the impossible standard they've set for themselves.
"It's chasing the dragon," he says, adding that as a result such people are often perfectionists and work extremely hard.
Young, who experienced Impostor Syndrome in graduate school, recalls her symptoms as "a nagging self doubt, an insecurity about my intelligence and abilities, about whether I could really do this and assuming nobody else felt this way".
Even today, despite her success, she occasionally experiences Impostor Syndrome.
"Whenever I step out of my comfort zone to a bigger spotlight or stage or a bigger challenge, I will have a flash of those feelings, but I have the information and the backup to put it in perspective."