I'll go first, because fair is fair.
I constantly avoided barbells. I was intimidated. I was afraid I'd hurt myself loading a bar I didn't know how to use. I was embarrassed by the idea of standing in the squat rack with just the bare bar while everyone around me was moving serious weight. And on top of all of that, I figured barbells were "just for bodybuilders"; so, I told myself I didn't need them anyway.
That last part was doing a lot of work. It let me dress up the embarrassment as a principled decision.
It took me an embarrassingly long time to realize I had invented a distinction that doesn't exist. "Bodybuilding" isn't a professional category. It's not competing on a stage in a Speedo with a spray tan. It's the literal description of what I wanted to do: build a better body. I'd been avoiding the most effective tools in the gym because of a word I'd misunderstood ... and because loading that bar with quarters (25s) felt like admitting something.
The nutrition one is worse.
I knew food mattered. I wasn't ignoring nutrition; I was just eating. Eating everything, tracking nothing, assuming volume was the variable that mattered. It isn't. I was optimizing for quantity and ignoring quality, then wondering why my outputs weren't what I expected.
"Eat more" and "eat well" are not the same instruction. I learned this the slow way.
What's the belief you held longest before something corrected it? Drop it below — the more specific, the better. Bonus points if it took you longer than it should have.