P. D. James, whose books I can recommend based on the fine appreciation of my wife who has actually read them, says this in her autobiography:

There is no point in regretting any part of the past. The past can’t now be altered, the future has yet to be lived, and consciously to experience every moment of the present is the only way to gain at least the illusion of immortality.

The sentiment about leaving regrets behind is common, so common it is likely taken for granted. Still, I can’t see how not regretting, rawly understood, is really of any benefit. Of course we ought not to live in fear or paralyzation or any sort of bondage to a past that has been confessed to God, but this does not mean abolishing all regret. It might mean better understanding something that happened, being thankful for an unexpected or unforeseen outcome and rejoicing in what was meant for evil being used for good. All of these healthy responses in no way mean that one shouldn’t regret doing evil if that is in fact what was done. If we’re not talking about evil, then refusing to regret makes sense since it’s simply another form of ingratitude or complaint over our limitations–“if only I’d known, then I would have turned left instead of right.” These regrets refuse to admit and embrace God’s sovereignty. Ethical matters, sins, are another thing. If we don’t regret these then we don’t realize the harm done, and forgetting simply ensures that we will not be matured by our past actions and therefore won’t react different in the future. This also leads to despair. Receiving forgiveness and learning from mistakes and sins is far different than just forgetting them. Leaving guilt behind but living in gratitude precisely because of all that has happened is the only way for reconciled people to love the idea of immortality–our past story and current adventures allowed to continue and come into more than we could ask or think.