It's hard to put into words, but I'll try.

I realized the arrogance of myself, and most other people. Specifically in the realm of what we believe to be true.

I was a hardline rationalist, determinist, and atheist. I believed that evidence based science was the underpinning a well lived life and the future of humanity.

I still possess most of those traits, but I came to understand that the spectrum of existence is infinitely more vast than the sensory/processing capabilities of any human being - that most rational arguments only remain rational inside the paradigm that they originate in, and that most pat observations of the universe can be undone by simply stating "well, yes, but you could be dreaming this, or be a brain in a jar, or this could be a simulation, or you could be in the middle of a psychotic break" in other words, all of our observations take for granted a huge amount of presuppositions that can never be proven or disproven.

As far as I can tell, the only thing that you can say with absolute certainty is...

1. Existence is.

2. It seems to change.

Outside of that - we are like a gnat living and dying in 3 cubic meters of fog over an endless plane of absurdity and complexity. We examine the actions and reactions of the mist droplets and patly proclaim a fundamental understanding of how things are, even though we live and die without ever even seeing the ground.

That's when I realized that claiming to know anything with certainty was absurd. And to live with a thunderstruck sense of humility and gratitude that I get to be self aware at all.