Believing with Absolute Certainty

My question is in regard to a recent question here entitled “How do I get over the feeling that people would think of me as Inadequate?”

Part of the answer given is:
“Whenever we have thoughts about what other people think, they are really just OUR thoughts. We can’t ever truly know what sentences run through people’s minds. Whatever we imagine is often just a projection of our own beliefs.

The only reason you are worried that the interviewer might think that you aren’t good enough is because some part of YOU believes that you aren’t good enough. If you believed with absolute confidence that you were good enough, you would not care one bit if the interviewer was judging you that way.”

Will you please expand on the Scholar going about believing with absolute certainty that they are good enough? Thank you very much.