I would say “regret” is another paradigm of human hypocrisy. The decisions or the actions we take, are a manifestation of our existence supported by circumstantial evidence. We do what we do, based on our perception of the world around us, that is our intellect along with a core will, constituting the basis of our existential purpose. Regretting our actions is only defying our existence and the true nature of our will. It’s well said that, ‘a man must do what he has to and live with it for the rest of his life.’ What the best one can do, is learn about the true nature of the will that governs one’s existence and come face to face with brutal facts of reality. By reflecting upon their actions, one can avail a chance at learning about this true nature and try making amends as far as the will allows, but who knows even learning along the way and changing our disposition may be a characteristic of one’s will.
Schopenhauer in his essays of “the wisdom of life” and “human nature” talks about in detail about his proprietary idea of will that implies “Will” in its integrity as a metaphysical object is incorrigible and can only be controlled by limitation to exposure with the help of intellect.
As far evidence and experience goes, people who regret their actions, are usually those who have really been so ignorant their whole life, when given the chance, time and again to face the true face of their existence. They just fail to see, or nay, lack the ability to discern, and they move on with their evidently repetitive action; when provided with similar circumstances but different situations, similar actions reveal themselves and then they regret when it goes wrong. People when talk about such actions in retrospect, express regret on the present manifestation of events as a result of their past decision. Instead of acknowledging their true nature, people express penitence on their actions, that merely demonstrates their hypocrisy.
People who can really change in accordance with their nature, depends on degrees of freedom provided by their essential constitution along with the conscious intervention of their mind. These features of an individual facilitates them in how they see things and handle them, that essentially depends on the nature of their intellect formed by a combination of both rationality and emotional intelligence. To discern the context of their actions with respect to circumstances and consequences. It is their intellect that has accoutered them with the ability to see and witness their true nature in action as well as from life experiences of others.
To know oneself is the quintessential lesson one has to get from this mortal life, else its meaningless and redundant. One must not regret their actions, instead accept them as a part of their existence and move forward learning to do better each time. Defying our true nature is in violation of this more than efficient perfectly coherent, correlated and self progressive universe.