People who dwell on regrets are making the error of granting the past too much importance.

The reason I say "too much importance" is because nothing about the past can be changed by anyone. It's self-evidently done, and over, by definition. Free will, choice and action can only occur in the present, as well as the near or long-distance future. Not so with the past. Therefore, to dwell on the past is a big mistake. The regret-filled person may counter, "But I must learn from the past." That's fine, but learning is not dwelling. To dwell is to stare. And to feel regret and remorse represents the end result of staring. Learning means to look objectively, at a distance, and as briefly as possible--for the sole and immediate purpose of making some kind of premise correction or behavioral change in the present and future.