Rejection involves the interpretation of an outside event. Regret is the rumination of perceived failed handling of an event or choice that is is now history. Both are common distress reactions. Neither is mandatory.

That said, I've seen rejection kill, and I've seen regret ruin quality of life.

Rejection reactions are more common and intense in relationships in the young. Regret seems to be more common in the daydreams of later years.

When rejection is simply the 'spin' one puts on failing to obtain a desired acceptance, or when regret fails to note that all events - within ones scope of power and outside it - are what makes one who one is, they are mistakes.

Mistakes because no one knows the future, and in order to be feeling one or the other, one is usually assuming a finality, and ignoring the possibility of an unexpected, or yet conceived, positive outcome.

Not to say that rejection by ones entire tribe, or regret at egregious harm done to others are trivial or not valid. But I can think of few sadder things the mind can do than to trap one in these things, wasting precious time. (yes, I've dabbled in both)