I used to believe that anyone, with hard work, can succeed and I realized that hardwork is no substitute for investment capital and something being on your life path. If its not for you, its just not for you and its important to gut check and know if something is what you REALLY want rather than something you are doing because its a good idea or makes financial sense.
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.