The only mistake I have ever made--and it is one that I have repeated over and over throughout my life--is not having confidence in what I suspect to be true. "Not having the courage of one's convictions" is too strong an expression of this failing, however. My suspicions rarely if ever formally solidify into convictions. Instead, my timidity or thoughtfulness or low self esteem (or whatever) prevents me from announcing/publishing my dimly sensed inner thoughts about something. Rather, I more easily adopt the ideas of others about things, because the holders of those contrary opinions are, in my view, more authoritative, more learned, and generally smarter than I. As a result, I suffer from a peculiar brand of intellectual depravity. For example, other than Yeats and Auden, I never cared much for modernist poetry. Yet, when I was an undergraduate at Columbia in the late seventies, modernist poetry was central to what was taught as poetry. So, I pretended to like Pound and Eliot more than I did. I suppose this is a common enough failing of undergraduates (at least in my day.) Yet isn't it the worst of sins? Better to cling stubbornly and foolishly to a genuinely held even if generally discredited belief. Not trusting one's inner voice leads over and over again into a peculiar brand of disappointment and confusion. It is disappointment tripled: first, one is disappointed in what turns out not to be true; second, one is disappointed in one's elected arbiters elegantiae; and, third, one is disappointed in oneself for having been duped. Attentiveness to one's own opinion leads not only to the dialectical testing of closely held assumptions and perhaps a habit of prudent investigation into those assumptions, but also to a journey of discovery about oneself. It is better to have loved and lost...
Of course, the above is a gross over-simplification. Nothing is more tedious or doomed than someone who only listens to his inner voice, who is tone deaf to the criticisms or insights of others. Yet in the crucible of one's innermost thoughts, there is an alchemy.