Have you read Mindset, by Dr. Carol Dweck? I'm convinced that impostor syndrome is actually a part of your personality that is fixed, that is to say, has a fixed mindset.
She advocates giving your fixed mindset a name, and then telling it to shut up every time she whispers in your ear! :-)
I'll have to check that book out!
I'm going to name my fixed mindset Edmond.
Nice, mine's called Nigel :-)
Cg