Failed one-person online businesses (and lessons learned)?

A recent thread discussed successful one-person online businesses (https://news.ycombinator.com/item?id=21332072)

I'm curious about the failed attempts, lessons learned, and what advice others would offer.

* How many people here have tried starting a one-person online business but feel that they have failed?

* What would you do better next time? What lessons did you learn?