How you deal with regret may have a lot to do with your age. A new study shows that young people have a much harder time letting go of regrets than the elderly, who don't spend much time crying over spilt milk:
A new study demonstrates that these cognitive differences manifest themselves in brain scans and physiological responses, revealing that, unlike healthy adults, both depressed adults and young people treat missed opportunities and genuine losses as equally regretful events—even if they were not directly responsible.
But the question is why don't older adults spend a lot of time in Regretville?
Although the new study does not offer direct experimental evidence to explain why healthy older adults in the study were less susceptible to regret, Brassen has a few ideas. She speculates that healthy older adults blame themselves less than depressed adults and young people, distinguishing between actions for which they are responsible and events they cannot control—such as when the devil in the game appeared. Brassen further proposes that disengaging from regret is a protective strategy that kicks in sometime in old age, preventing the elderly—who do not have as much time or opportunity to make amends—from needlessly feeling sorry about things they cannot realistically change. In contrast, young people have their whole lives ahead of them—plenty of time to repeat their mistakes if they do not learn from them.
The brain is an evolving process that continuously refines itself. This refining involves things like perfecting certain motor movements as well as managing interpersonal affairs and subjective states. To assume that people would use the same self-regulatory strategies from birth to old-age is to overlook the evolutionary nature of the brain and mind.
There is evidence of a decrease in self-referential thought corresponding to a subjects age. Older people are less likely to ruminate on self-referentiality the way that teenagers do. The teenage mind is almost entirely consumed by self-referentiality, whereas the aged mind is relatively absent of self-referentiality.
Self-referentiality refers to states where thoughts of how one fits into the world are evaluated relative to an ideal.
This adaption reflects an implicit progress of the mind toward a self-less world-view. A perspective that does not view the self as a substantive part of the world. The elderly brain may view its failures as a consequence merely of an elderly brain and hold no higher or lower regard with respect to the self. Rather than perceiving its actions as issuing forth from a contra-causal and disembodied entity deserving of cosmic approval and disapproval; it views it's failures and successes as probabilistic occurrences of a diseased brain.
There is a "spiritual" (philosophical) tradition which views all of self-conscious life as a progression along this path toward a correct interpretation of the world which views human thoughts and actions as a continuation of the world-process (aka The Universe, Nature, God, etc...). As a basic premise of this tradition we are assumed to be born relatively ignorant, by a young age a flawed ego-centric cognitive structure takes hold which characterizes most of our lives. Though we are primarily trapped in an ego-centric cognitive structure we progress toward the eventual realization that the ego is false. Though this realization is not necessary and indeed most will never have it; everyone will progress in some degree toward this end.
Additionally, the ego cannot bring itself to the realization that it is false. So there is nothing that a person can self-consciously do to convince themselves that they are false. Rather, the world-process ("the unconscious") does all of the work furnishing the conscious mind with it's content. The ego is a product of the world-process which provides the basis for the appearance of things. So people, though they progress toward the end realization of the falsity of the ego, are completely unaware that they are doing so.
In other words; older people stop ruminating on regret self-referentially because they implicitly realize that it is not True. That doing so does not reflect the actual state of the world. They may still experience regret and it may still work its magic altering future behaviors, but it's not descriptive of a subjective ego and can be dropped afterwards.
Check out: The Handbook of Personality and Self-Regulation, The Handbook of Self and Identity and perhaps Johann Gottlieb Fichte's The Science of Knowledge. There are very few philosophers who describe this position, even fewer who do so well.
Life is too short to dwell on things that are both 1. in the past and therefore can't be changed and B. unpleasant.