Imposter syndrome is not an official diagnosis, but it is a mental phenomenon where you consistently doubt yourself. It is hard for you to appreciate your own skills. Other people might even point out your accomplishments and how hard you worked to earn them yet you believe it is due to coincidence or luck.
You attempt to overcompensate by placing expectations that are hard to reach. While you try to attain these expectations, you criticize and sabotage yourself. When you do accomplish the high expectations you’ve put on yourself, you believe you do not deserve the recognition. You continue your efforts to achieve more and push expectations to impossible standards. Thus, the cycle of imposter syndrome is put into place.