Non-clinical depressive realism

Regret is the emotion of wishing one had made a different decision in the past, because the consequences of the decision were unfavorable.

Regret is related to perceived opportunity. Its intensity varies over time after the decision, in regard to action versus inaction, and in regard to self-control at a particular age. The self-recrimination which comes with regret is thought to spur corrective action and adaptation.

In Western societies adults have the highest regrets regarding choices of their education.

Source: Wikipedia

People may be deeply troubled by the lost lives they could have led, and the lost selves they could have been, if they had acted differently in the past. It is associated with the primordial fear of being a loser. For that reason people are loathe to admit regret.
Scientists claim that regret can be a form of creative thought, motivating people to improve upon their lives. Those harbouring regret tend to perform with greater accuracy in judgement and process information more analytically than others.
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(F) Fuzzy exceptional problems