I dropped out of a PhD program because of funding cuts and, after a few twists and turns, got a job in a research division of IBM. I was 1 million percent certain I was in the wrong place -- these were really smart people who were very seasoned software developers -- people who had invented programming languages and operating systems.
But I felt the same in grad school, and, to a certain extent, as an undergrad.
I got a really good yearly review, a pay raise, and more responsibility and thought, "My boss is just being nice. If he really knew .."
Yeah, they promoted me out of ignorance -- that MUST be it.
Decades later I realize I will never be done with that sense that I am in over my head, it is always tapping me on the shoulder and reminding me that what we do is hard and someone is better at it than me. But I've done ok and I keep coming back to the question of "why do I do this at all?" Because so many people get their pride caught up in the profession like I do and feel a certain sting when things change -- be it more or less responsibility.
I've come to the conclusion that there is a certain craving at the bottom of my impostor syndrome, a craving for validation that both motivates me and criticizes me. That group at IBM was slowly dismantled and I was shown the door like everyone else. It was crushing, I still mourn that loss, and I still feel that sing of doubt. If my craving for validation was amenable to reason, this would have passed long ago -- so it is unreasonable and is no longer helpful so the best I can do is recognize it and talk back to it.
I had a psychotherapist (old school Freudian talk therapy, great stuff) who had a quaint office filled with books and a funny little stitch pillow that said, "If all else fails, lower your standards". On our first meeting he pointed to the pillow and asked "What do you make of my little stitch pillow?" I laughed and he smiled and explained there are only ever two reactions to the words -- laughter and fear. The pillow is something of a litmus test. People seeking therapy are all working out how adjust their standards and if they will be swept away by unseen forces if they can't be flexible. But I believe everyone is working that out -- how to be flexible so they don't get swept away by rigid standards -- especially the standards they have adopted to help motivate them in the first place.