Confidence without knowledge is just a nice car with no engine.

Certainty in your manner and competence in your hands are separate things, and only one of them is easy to fake. Confidence gets you the room and cannot do the work, so the gap appears the moment somebody asks a second question. The trouble is that both feel identical from the inside. You are the last person to find out.

Notice where you are performing sureness about something you never actually checked. Say the honest version, that you think so but do not know, then go find out. Confidence with the thing behind it does not have to be maintained.