While Imposter Syndrome is not a mental illness or disorder in and of itself, it is often related to Social Anxiety Disorder and is an example of disordered thinking that can a symptom of a variety of other mental illnesses. Wil Wheaton talks about it in his keynote address at the 2016 Mensa gathering.
"The way it works is this part of my brain that’s supposed to be on my side but is really a dick about everything goes, “You know, you suck at everything and you don’t deserve to be here and nobody likes you because you suck. Boy do you suck. You are the suckiest bunch of sucks that ever sucked.”"
Even though there is plenty of evidence to the contrary - good job, loving relationships, nice house, great pets, etc - there is a constant feeling of being a fraud or that somehow you don't deserve success. Your brain is being irrational and it can be very hard to see reality.
I really encourage you to read Wil's post, because he explains it so much better than I can. You can do a search for Imposter Syndrome to start at that section, but the whole thing is worth a read too.
"Depression lies, and when it tells you these lies, you can look right back into its stupid face and say, “Shut up. Wil Wheaton told me that it’s okay to get help, and he pretended to live in outer space, so he outranks you.”"
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