Ahh, you have raised one of the oldest questions in epistemology (the theory of knowledge). If you are seeking some unassailable foundation for all of your knowledge, then for the reasons that motivate your question, you are going to fall short. For any foundations you propose can be queried for justification: Why believe them? If they have no justification, they don't constitute knowledge; if they do, you are off on a regress. Or, to put it a different way, if you had a criterion that would tell you when you know something and when you don't, you'd have to justify that criterion. But if you used that criterion itself as the justification, you would argue in a circle, and if you needed another criterion, you are off on a regress. Sextus Empiricus, in THE OUTLINES OF PYRRHONISM and AGAINST THE LOGICIANS among other texts, developed such arguments in great detail. Nagarjuna, in VIGRAHAVYAVARTANI, develops interesting variations in an Indina context. He argues that we can only test...

Because sometimes your life depends on it. (E.g., there is a monster under the bed, or there is a panther behing that tree, or that guy is going to turn left right in front of my car.)

A complex question. Being sure can be read as a psychological state, a feeling of complete confidence. And of course we are, as a matter of fact, completely confident about a lot of things. At least I am. But the real question is whether we are entitled to that feeling. That raises the question of what kind of entitlement is relevant. Is it pragmatic entitlement? That is, does such confidence yield good outcomes? It seems to. Or would it require proof beyond the possibility of refutation? The latter raises hard questions about what proof is and about what refutation is. What would a starting point for a proof be? How would you justify the form of argument you use? But how would you decide what to take seriously as a refutation? You can see that this is not a simple matter. I suggest that you read Sextus Empiricus, OUTLINES OF PYRRHONISM and then later, Wittgenstein's ON CERTAINTY to get a feel for how much fun and how hard it is to think these questions through.

You should believe nothing I say, including this.