It's a commonly cited figure that at least 90% of people who commit suicide have a diagnosable mental disorder, and here's a paper that claims the figure is as high as 98%. Of course there could be some tautology in the diagnostic methods, but suicidality itself isn't classified a mental disorder.
It bothers me that this fact is usually interpreted to mean that suicides are the result of poor judgment or a disconnect with reality. Mental illness is a common cause of genuine severe suffering.
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.