Can you describe something in your past that you really regret doing?

I suppose there are turning points in all our lives. Sometimes we learn a lot from them; sometimes we just feel a lot of regret. These things might haunt us, keep on coming back to us, reminding us of that regretable situation, making us feel bad about ourselves. Perhaps it is a failure of some kind. Perhaps it is something completely out of our control. Perhaps it was waiting too long to do something. Whatever it is, can you describe it and tell it’s story?