Well finally science has something to say about this. A study: "How unrealistic optimism is maintained in the face of reality" by Tali Sharot, Christoph W Korn & Raymond J Dolan published in Nature Neuroscience (2011) has some fMRI proof that these behaviors are hard to change.
In the study the authors find:
"Unrealistic optimism is a pervasive human trait that influences domains ranging from personal relationships to politics and finance. How people maintain unrealistic optimism, despite frequently encountering information that challenges those biased beliefs, is unknown. We examined this question and found a marked asymmetry in belief updating. Participants updated their beliefs more in response to information that was better than expected than to information that was worse. This selectivity was mediated by a relative failure to code for errors that should reduce optimism. Distinct regions of the prefrontal cortex tracked estimation errors when those called for positive update, both in individuals who scored high and low on trait optimism. However, highly optimistic individuals exhibited reduced tracking of estimation errors that called for negative update in right inferior prefrontal gyrus. These findings indicate that optimism is tied to a selective update failure and diminished neural coding of undesirable information regarding the future."So, is this scientific challenge to your ability to get better at estimation enough for you to quit tracking actual time spent on stories/tasks? The evidence is that your bias will keep you from learning except in the cases that prove you are faster/better/more awesome than you estimated you were. Which leads many managers to decide that there teams are sand-bagging and just not as good as "we were in the older days," a different human behavior to study.
Your Brain won't allow you to Believe the Apocalypse could actually happen