Could I be wrong about everything I know? Yes. Of course. This is what epistemology is all about - trying to answer the question, "How do we know what we know?" The bad news for anyone claiming absolute certainty is that there is no way to prove something absolutely. "It's turtles all the way down, son," as the saying goes.
Every claim must be supported by a prior claim that is proven to be true. But, working your way down, you find that all the claims ultimately rest on the point of a single, unfounded assertion that is treated as an axiom, like an inverted pyramid. So then you realize that you need another way of judging a belief structure, and most philosophies rely on the coherence and integrity of the belief structure. The more that each claim is networked to other claims, reinforcing each other, then the more likely the belief structure is to be "true."
The main reason most people don't bother to examine their beliefs is because of what some have called the "previous investment trap." A person has lived their entire life up to that point with a set of beliefs. If they find something that challenges their beliefs, they are likely to reject it, because changing means that they have been wrong all this time, so they feel their life had been a waste. How willing a person is to change their mind is related to how much they value the past. If someone believes that their past gives their life meaning now and in the future, they are unlikely to accept changes to their belief structure that render their past choices and beliefs as wrong. However, if people adopt the ideas that the past does not equal the future and that, no matter how hard you try, you cannot change the past, then they are more likely to accept new ideas.
Also, if someone can see that the new belief structure can be as safe and supportive as the one they have now, they are also more willing to change.