Regret is probably there to get you to relive a past event where you made a wrong choice, while feeling bad about it, so that you learn not to make that mistake again.
As a consequence, if you think you might make the same bad decision again, reminiscing and feeling regret might continue to be helpful. Maybe there was a nuance to the situation that made it unclear whether it was a bad decision. Once you know the decision was bad, and know you wouldn't make the same bad decision again, regret is unnecessary; from then on if you catch yourself in it, you should redirect your attention to something productive.
As a consequence, if you think you might make the same bad decision again, reminiscing and feeling regret might continue to be helpful. Maybe there was a nuance to the situation that made it unclear whether it was a bad decision. Once you know the decision was bad, and know you wouldn't make the same bad decision again, regret is unnecessary; from then on if you catch yourself in it, you should redirect your attention to something productive.