It would be impostor syndrome, if we were excellent, but fearing that we are frauds. But if we are in fact frauds, then "impostor syndrome" is the natural self-diagnosis self-defence protecting us from admitting to ourselves the bitter truth. Maybe it's a better idea to react to the feeling of being an impostor with the will to change something in one's life? I know I feel much happier knowing that I have a plan which I systematically put into life, that lets me do fulfilling stuff, instead of the usual theatre.