This state of mind is really interesting and super, super common, especially in high achieving professions, and for women. I've nicked this definition from the Geek Feminism Wiki, because I love it:
"Impostor syndrome describes a situation where someone feels like an impostor or fraud because they think that their accomplishments are nowhere near as good as those of the people around them. Usually, their accomplishments are just as good, and the person is applying an unfairly high standard to themself (and not to others). It's especially common in fields where people's work is constantly under review by talented peers, such as academia or Open Source Software."
Symptoms of Imposter Syndrome
It's what it says on the tin - you feel like an imposter. When you are at work, most commonly, but it could be in a group of friends or your community, school, or any other social location. You feel like you do not deserve to be there, and that your achievements are less valid or less worthy than your peers'.
This leads to the individual feeling as though they are going to be "found out", and struggling with chronic feelings of inadequacy. Over time this can be demeaning to confidence and to a person's sense of self-worth, particularly if they are in an environment where they don't receive much support, encouragement, or reward for their work.
Drawbacks of Imposter Syndrome
Sometimes imposter syndrome can be paralysing professionally. People often report they will not write or submit papers, assignments, projects etc because of the fear that their colleagues and superiors will realise that they are not as good as they are meant to be. They might not even apply for jobs which they are eligible for. If they do they may understate their achievements on their resume/CV or to potential employers. They may struggle with a competitive envornment. All of this anxiety around one's own place in the world professionally is exhausting. People who have imposter syndrome anecdotally have higher levels of stress.
Benefits (!) of Imposter Syndrome
It may feel very difficult to cope with, but having imposter syndrome certainly can be used to one's own advantage. These individuals tend to prepare more - a bonus in any field. They are not arrogant and do not overstate their ability, and are therefore safer and more productive. They are notoriously hard workers and tend to be highly valued. They also are more diligent and easier to get on with in a team environment.
So, sometimes you feel like an imposter. It is natural - up to 70% of people experience this at some point. It has benefits and perks. There are lots of tips around on how to deal with this - use of language, confidence building tips, and many more. You can absolutely use this to your advantage and make it a positive aspect of your life.