There are lots of different things, all of which are called regret. I treat them differently, so I need different names for the different classes of regret.
Agentic regret. The sort of regret that you feel when you made a choice that was wrong and you knew it was the wrong choice and you failed as an agent. You knew you should have taken out the trash bins, but you didn’t, and now it’s the next day and you have full bins and no way to deal with them until next week. Hope you don’t need to do anything that produces trash.
Bad-randomness regret. You made a choice, and you knew it was a gamble, and you lost, even though the gamble had positive expected value. Even if you know your action had positive value, if you had the information while you made the decision that you had now, you could have made a better decision. If you could have gained the information and you knew that ahead of time, this is agentic regret—failing to gather the information required to make a good decision is a bad decision.
Self-knowledge regret. You made a choice, and got what you thought you wanted, but it turns out you didn’t want that thing. This may be revelation regret if you knew that you didn’t know what you wanted. But sometimes this comes as a surprise—you get to the top of that mountain and think “whatever, this sucks, I want to go home”, or you finish that book you’ve been forcing yourself to read and think “whatever, this sucks, I don’t want to be at home”.
Temporal-conflict regret. The version of you in the past that made a decision (warning: real philosophy happening, “what is the self?”, panic and flee). That decision was good for them but is bad for you, the version of you that exists now. In other words, some asshole (your past self) did something unkind to you (made a decision that benefitted them at the cost of you). Procrastination is a good example of this. If you spend a day on disposable pleasures at the cost of avoiding your meaningful pursuits, the you who has to clean up … your …. mess will be upset with … you … or whoever. Is this agentic regret? Not always. If you plan to spend the next two days doing one day of work and one day of leisure, it is selfishness, not failure, that causes the you of now to saddle the you of the future with the day of leisure. (Or is it selfishness for the you of the future to demand the day of leisure over the day of work?) Regretting a relationship after a breakup is often this: the you of today has to deal with the fallout of the breakup, but the you of the past got to enjoy the relationship while it existed, the selfish bastard.
Regret is a form of suffering, it is a negative-valence emotion, and negative-valence emotions are only worth feeling if they motivate choices that you endorse.
Agentic regret is worth feeling, because knowing that you will suffer if you choose poorly motivates you to choose wisely.
Bad-randomness regret is worth destroying, because knowing that you will taking positive expected value gambles motivates you to miss good opportunities.
Self-knowledge regret is worth pondering, because the value of learning more about your utility function is often worth much more than the bad decision, but the cost of working towards the wrong goal is also often very high.
Temporal-conflict regret reflects a lack of unity between your selves. It is worth getting to a point where you are unified in your desires and you understand, on a level where it has become an inseparable part of you, that every action you take represents a policy of always taking that action in that situation. Then make policies that minimize inter-self competition.
Feel the regrets that it helps you to feel, and have equanimity about the situations that would otherwise cause regret that is unhelpful.