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