There is an actual Imposter Syndrome that afflicts people who are highly qualified/credentialed, people who have qualified for entrance into elite programs, and put in the time and effort to progress to graduation; and yet for psychological reasons, they have great self doubts about their "worth". This is akin to depression. (a third party would look at you and say "wow, you are super competent and qualified, it's terrible you feel you aren't," and no pep talk they give you is going to help you.)
Then there are bouts of insecurity and guilt whereby any of us can feel like "I fibbed on my resume and interview to get this job, and I now I'm surrounded by people who know what they are doing, I hope nobody figures out I snuck in." This is akin to sadness. (a third party would look at you and say "ha ha serves you right, but it's always fake-it-till-you-make-it, everybody feels this way to start out, just grit your teeth, you'll be fine," and that pep talk will help you.)
Everybody experiences sadness; sadness clears up and goes away. Not everybody suffers clinical depression, and it does not go away on its own.
Dunning Kruger? it's not really an Effect, it's an observation/tendency that is explained by simple logic along the lines of, "you don't know what you don't know".