As a teen I was heavily into "psychic" phenomenon. To the point where I'd even managed to convince myself that I was psychic. Even getting actual hallucinations with out of body experiences. And then I read a book by susan blackmoore, a former parapsychologist. She introduced me to common sense experiments with actual strict methodology behind them. Surprise ending, I realized that not only wasn't I psychic, nobody was!
Absolute loss of the foundations of my worldview. Sure, I wasn't very old. But this had been something that was the center of how I'd perceived the world for a pretty large part of my young adult life. To the point of influencing not only what I did, but who I hung out with, dated, etc. To this day I'm pretty proud of young me for going with the truth instead of retreating back into what would have been a far easier path of denial.
There was some good and bad in that. The bad was that I lost some friends, and the girl I was dating at the time. The good was that it influenced me to get into psychology after graduating. With a heavy focus on learning just how the human mind manages to fool itself in situations like that.