"We want you to be branch manager." I hung up the phone, those out-of-the-blue words still ringing in my ears. It was 2006, and my department was being outsourced to another financial services company. And now apparently they needed a branch manager. I was thrilled - for a whole millisecond. Then the doubts started creeping in.

"They just need a branch manager for legal reasons and I'm the most convenient person to ask. It's probably in name only. And what happens in a few months when they realise they've made a mistake and I'm not cut out for it?" said the little voice in my head.

This is Impostor Syndrome – the unfounded fear of being found out. Sufferers don't attribute promotions and achievements to ability; they assume somebody somewhere has made a mistake, and sooner or later, there will be a hand on the shoulder, and a voice saying it was all a mix-up.

So how do you know if you have Impostor Syndrome?

  • You think your successes are down to luck or error, rather than ability and hard work.
  • When you are promoted, you worry that it's unwarranted.
  • When peers praise you, you assume they're overestimating you or are being fooled into thinking you're better than you really are.
  • You find criticism difficult to take, even when it's constructive, and you tend to over-analyse, focusing on the negatives.
  • You may shy away from challenges because of self-doubt.

And what can you do about it? The first step is to recognise it. "Yes, before anything else you need to identify it – label the thoughts as they trickle into your head," says Gillian McGrath, life- and business-coach and master trainer (ChangeGrowSucceed.com). "Say to yourself 'Here's the impostor bit – I'll give it its space'. By acknowledging it you're creating a bit of distance, and then detaching yourself from it."

She also says we should avoid unrealistic comparisons with other people. "I had a client who was attending a presentation and comparing herself to the very confident presenter. My client was thinking 'I could never do that'. But in that scenario, what you're essentially doing is comparing your inside to another person's outside, and that's an unfair comparison. You don't know what's going on for that person inside – she could be petrified. It's not comparing like with like."

Another tip is to note your achievements. In my case, I took a colleague's advice and created a "HAIG" folder – it means "Hey aren't I great!" Mine was an email folder, into which I put all correspondence that made me feel good – thank you notes, targets met, performance reviews, and praise of any kind. McGrath agrees that writing down the positives is a big help. "One client of mine found journalling very effective. She had so many qualifications and accomplishments, but still felt like a fraud. So I suggested she buy a notebook, leave it at her bedside, and at night write down events in her life that had led her to this point in her career – always focusing on the positives. Over time, her feedback was that she felt much more positive, and for her it was the journaling that worked - because it was a daily habit and it suited her."

Mostly, despite worrying from time to time that I was a fraud, I got on with my work and didn't find Impostor Syndrome debilitating. But could it cause serious problems if left unchecked? Gillian McGrath thinks so. "There's a fine line between a moment of low confidence and feeling it all the time. I've had some clients who feel it constantly and what they're presenting with is stress and anxiety, and some of them are less likely to explore new opportunities as a result."

The irony in all this is that Impostor Syndrome is usually the domain of high achievers, whereas people who really aren't capable tend not to be aware of it. In fact, there's an opposing state called the Dunning-Kruger effect, whereby low-achievers have an illusion of superiority. So the next time you wonder if you're really cut out for that promotion, remember that a little bit of self-doubt is preferable to a false perception of supremacy.

I no longer work in financial services; after 17 years, an offer of redundancy came up and I took it. I moved into freelance writing, then fiction, and Impostor Syndrome came along for the ride, though I find that self-employment helps mitigate the self-doubt; as any freelancer knows, nobody hires you just because it's convenient or you're next in line.

The little voice still pipes up every now and then, but I'm better at shushing her. Plus I have less time to overthink than ever before. "People with Impostor Syndrome sometimes live in their heads," says McGrath. "But step back and focus on the value you're bringing instead of giving so much volume to those internal thoughts. We all want to feel confident – that's universal, but it does involve tackling those small monsters in our heads."

So if any of this rings true for you, stop waiting to be unmasked – chances are, your boss already knows the real you, and that's exactly why you're where you are today.

Even celebrities can have Impostor Syndrome:

Tina Fey mentioned feeling like a fraud in an interview with the Independent newspaper: "The beauty of the impostor syndrome is you vacillate between extreme egomania, and a complete feeling of: 'I'm a fraud! Oh god, they're on to me!'"

Hugh Laurie talked to Variety magazine about his Hollywood Walk of Fame star, saying, "There's joy and there's a strong dose of impostor."

Jodie Foster felt like a fraud when she won an Oscar for The Accused, telling CBS News "I thought it was a big fluke… I thought everybody would find out, and then they'd take the Oscar back."

Bruce Springsteen spoke on The Late Late Show recently about going through periods of self-doubt; feeling like a "charlatan" and a "fake".

And even Sheryl Sandberg has said there are days when she wakes up feeling like a fraud, not sure she should be where she is – we are in good company.

(Article first published in Magpie magazine)

PS One final note: Impostor can be Imposter but the former is more correct. This bothered me so I had to check. Now you don't have to!