The most important thing I know: no absolute truth. I guarantee you that something we believe right now is wrong.
Mathematics seems so pure it should have immunity to errors. But what if a proof has an error so subtle, the smartest people in the world can't find it? There's a (wikipedia page)[https://en.wikipedia.org/wiki/List_of_incomplete_proofs] listing exactly those cases. Some of the errors persisted for dozens of years, a few for hundreds. Computers can't make the problem go away, they have to be programmed by people.
Some say religious texts hold absolute truth. I won't argue anyone's faith; but I will counter that any holy text has to be interpreted by profane, flawed, ordinary people.
If absolute truth exists, then it can only be known at the end of time, when every last fact is finally known. Some things we feel sure of right now, we will find out in the future are not true. There's no way to tell.
This concept can bother people, but paradoxically, accepting it actually leads to a more accurate view.
Unlike Mathematicians, Engineers work exclusively in the messy physical world. They know, like truth, a service can never be completely relied on. 100% of anything promises a false sense of security. Even 99% reliability isn't great – a service that's 99% reliable is unavailable 4 whole days per year.
Instead, engineers look for "five nines": 99.999%. Accepting the possibility that something could change frees you to deal with it.
Changing your view to five nines will change your life.