There is no point at dwelling on things, the actions have been made and they can never be undone. All we can do is move forward. Some regrets can be remedied – it would be a lie to claim that every mistake can be resolved, for a lot of errors we need to accept their facticity – some things have happened, some people will forever be reluctant to overcome grievances.
Some things are just beyond the possibility of resolving, like your last words to somebody, not knowing that it would be the last time you would ever see them. Moments like this are something that stays with us, you cannot repress them. You can’t take them back either, you cannot change them, you cannot escape them. So what do you do? Nothing. Life is not fair, life is not blissful, it is unstable and subject to radical change without notice. All we can do is learn from the mistakes we make instead of repeating them in cyclical fashion, to break free from the wheel and escape it. We get caught in this cycle of ups and downs, we forget that repeating the same actions that lead to disaster before will always bring back the occurrence we are trying to escape.
This may sound nihilistic, call it that if you desire – but sometimes the nihilistic view is the realistic view, sometimes the truth is bitter and leaves a bad taste in your mouth. The truth can be repellent to the daydreamer. Utopian ideas are always better in theory than practice.