Regret is a big word. The dictionary calls it a pain of the mind on account of something done or experienced in the past, with a wish that it had been different; a looking back with dissatisfaction or with longing; grief; sorrow; especially, a mourning on account of the loss of some joy, advantage, or satisfaction.
But we attach more meaning to words. They mean more than just a dictionary definition.
Regret is an all-pervading feeling. It swamps you. You regret things that never were. Things that never should've been. Things that can never be. You regret opportunities lost; people hurt; things said.
But the worst kind of regret is for something that made you happy. A moment that was yours. You wonder how that is possible. Well, it is. you regret a moment that you lived for you, because in the process you opened up a can of worms that should've been left closed. In that moment you let loose emotions that never should've existed. And you broke down walls that should never have weakened. Your moment.
The dictionary defines regret as pain and grief.
Regretting a moment of happiness is the most painful type of regret there is.