Honestly, learning to program. I was listless when I got into university, and couldn't cope with the fact I wasn't the smartest person I knew anymore. That hit me a lot harder than I expected, and cascaded into a complete loss of confidence.

Learning a skill from the ground up I couldn't bullshit from random esoterica I'd picked up from high school forced me to confront my lifelong hubris and put aside my ego. Eventually I got decent enough at it that it became my career. It was a slow, grinding, painful process, but it taught me a lot about humility and the value of incremental progress.