There are a few things most people look back in life and wish they could have done differently - there are also those who can look back on their lives and say if they had to do it all over again, they wouldn't do anything differently. I am not one of those, I've got the share of regrets.
As an immature 21 year old, who refused to spend Christmas with an unwell grandpa, tops my list. The following summer I learnt it was Grandpa's last Christmas. Regret taught me quite a bit on a single day. It did enhance value & importance towards people who matter. It helps mature, be stronger - yet remain humble and softer in this short visit called life.
The more I think about it, the more I realize regret is perhaps a forgetful, subtle yet powerful emotion. People hardly share regrets with similar intensity as love, hate, anger or frustration. That said - regret is instructive for you to a point. After that, it becomes self-punishment. What's done is done, and there's no escaping that fact, have got to move with the experience, which leads me to believe, regret is remorseful, yet possibly a melancholic success.