When I worked the 9-5 I felt like a fraud quite regularly, often wondering when I would finally get the sack and imagining the sigh of relief when I got discovered. No more pretending, no more having to look like I know what I am talking about, finally outed to being the fraud I was.
When I set up my own business this feeling went along with the belief that I wasn't good enough, but I started with some support in the form of my own personal breakthrough and I soon realised that the feeling is with many people, from all walks of life, employed or not. This is often a link to a limiting decision at the unconscious level but having a conversation with a friend recently we talked about how she was doing what she loved, more than anything else, and not only that, but people were PAYING her for her opinion, which she often found amazing.
So many people believe that what they know is what EVERYONE knows, so it's common knowledge, it's easy, why would anyone pay you for it?? The problem is that what is 'common sense' to you, may not be so common. Maybe what is less common is confidence, confidence to take what you know and charge money for it.
When I took the leap from employment to self-employment I remember people asked me what I was going to do, where was I going to go... they were incredulous at the idea of taking a leap with no safety net. "But, what if it doesn't work?" they'd say. One even said to me, "you're not supposed to enjoy work though are you?" Some where along the line we learnt that we don't get paid for what we love to do, work is supposed to be hard, boring, difficult, challenging.... we roll our eyes at the idea of work, we don't wake up excited and we definitely don't get paid loads of money for something we find easy... or do we?
A friend of mine worked really hard in an administration role, and with every opportunity put herself forward for promotions, even though she didn't think she was qualified for the roles. Eventually she was noticed as a go-getter, ambitious and passionate about the company she worked for and within a matter of months she was one of the companies top earners. Now she says she feels bad that sometimes she works from home and does her washing and shopping instead of the work she is getting paid for (on other occasions she works evenings and weekends without payment) - the simple fact is, she finds what she is doing now so easy and can't believe the salary she gets for it, thereby worrying that she gets too much for not doing enough.
Perhaps the feeling of 'imposter syndrome' is a way to give you feedback, perhaps you do need to deal with an underlying belief that you are not good enough or don't deserve it (or some other limiting belief that can be just as stressful) and on the other hand, perhaps you have worked hard to get there, and perhaps the knowledge, talent and skills you have are simply everything that someone else needs.
Perhaps you are pretending to be someone that in fact, you already are.