Late into the discussion, but I feel my experience might be relevant.
I went through several high-tech startup episodes, each one failed due to its own “objective” reasons. In hindsight however, I have come to the conclusion that I squarely sabotaged them, despite having founded them, creating mostly brilliant technology and experiencing huge business prospects. I repeatedly found myself in a situation as described by the OP. Not quite a feeling of “impostor”, rather one of “usurper”: of un-deservedness, un-familiarity, definitely “not feeling well in my clothes”, and some hint of futility.
As irrational, absurd, pointless, crazy to outside observers (as well as to “the other part” of ourselves) that little voice may be, it’s there and it won’t let go.
Through those years I had recurring, nightmarish dreams, all a variation of a basic theme, conveying a message like “you have not finished your schooling; you have not learned all the lessons”.
At some point, tired, uneasy, depressed while in the middle of crafting a 5M$+ deal I decided to kick it, let it play out, taking a leave of absence without any self-imposed time limit.
After three years, having lavishly spent all my savings I found myself broke and down-and-out.
Then it finally, slowly began to dawn. Long story short, I had all but forgotten the metaphysical side of building a temple, the synergistic connection between what you build in the real (outside) world, and what you build within you. The two need to go hand in hand, because once you are swimming in money, it becomes virtually impossible to climb that other hill. .
Confronted with such a fundamental dilemma, bits of advice like “Fake it till you make it!” entirely miss the point, and don’t cut it. And “Don’t let that little voice in your head be the one that stops you”? Actually, I will be eternally thankful to that little voice in my head.
What have I learned through this, at times extremely painful process? It’s philosophical as much as psychological; most people won’t care, many will find it laughable, but here it is again, rephrased:
Although furiously celebrated as the ultimate bliss and cure-all in western culture exemplified in the American dream, material wealth by itself is not much more than a metaphor: it becomes an empty shell unless it is built up in tandem with interior progress and wealth.
More precisely still, the interior masonry must take precedence; procrastinating it won’t work.