Everyone has regrets, the difference is how you let them affect the way you live your life from that day forward. It's great to learn from them, but to dwell just puts you in a state of depression.
I regret not being serious at school. Took too much time goofin' around, content with passing classes, and wasting a year. It didn't have any bad side effects or anything, but I just lost a year, you know?
I regret not working hard on a relationship with an ex. It could have been something special, I'm sure we would be married by now. But I just regret how the break was never clean, just more assumed. It's the cliche now about us being two very different people now, we still get along, but we're now dating different people, and in our own different stages of life. Rather than wonder what could have been, we're looking to what we have now.
And one more, I regret not spending more time to travel before I started my path up the corporate ladder. Nothing more relaxing that travelling the world with no obligations.