Oh, I love this question. Because when I first started, I didn't just believe one myth—I built my entire early strategy around a glorious, catastrophic lie.
The biggest myth I believed was:
"SEO is a one-time setup. Just 'optimize' the site once, and you're done."
I treated my website like a piece of IKEA furniture. I thought: "Alright, I’ve installed the SEO plugin, I've written my meta descriptions, I've added my keywords to the H1s, and I've submitted my sitemap to Google. The furniture is built. Time to collect the rankings!"
I would literally check my rankings the next day, confused why I wasn't on page one. When I didn't see results in a week, I'd assume I'd done the SEO "wrong" and would start tweaking titles again. It was a frantic, reactive mess.