I realized I was asking everyone around me for permission to be there, and that was undermining my confidence. I don't need anyone's permission to do my job. I'm the real deal, not an impostor.
Like the spy movie cliché, people tend to assume you belong and you know where you're going. You just have to act like it. Do it long enough, and you might just fool yourself.
So when I feel uncertain now, I just act. I pretend confidence, and the confidence becomes real.I didn't know about Impostor Syndrome until late after I left graduate school; by that point, I had passed through huge swaths of it, to the point where I spent days at my hood at my new institution, wondering when my supervisor would come to escort me out of the building and when my badge would stop working.
Of course, in the 5+ years since, I've realized that impostor syndrome is incredibly common and even happens for people who have been well-established in their careers for years and years. To an extent, I actually wonder if lack of it is a sign that you've gotten into a rut. I dunno, but I still feel like one now and again. (I've managed to fool all of you. No you haven't. - ed.)