Regret is an emotional word that may originate from a very old word meaning to cry or weep.
You can regret something you did or, something you didn’t do. They say that in older age regrets for inaction can be quite strong: e.g. why didn’t I study harder at school, or why didn’t I spend more time working on my health and physical fitness, which can lead to feelings of wistfulness (one of my favourite words).
Perhaps the hardest regrets to bear are those where you can’t shift the blame and can only squarely blame yourself.
But I do think that it is possible to regulate regret in some instances. Take the two examples above. In retirement you will have the TIME to become fit and/or to study. Yes, it may have been better to have done these things years before, but you can choose to engage with them even now. The obstacles to achievement may not even be as large as they were before.
Retirement: You won’t know what it is like until you get there.