Maybe you regret not working hard enough in high school or college. Perhaps you wish you never turned down a person or an opportunity, or maybe you can't move on from a bad thing you said to someone.
Regardless of what it is, it won't help you to keep thinking about it. I understand that it's hard, but you must learn to forgive yourself for the mistakes you've made, rather than torturing yourself with the what-if questions.
Feelings, such as guilt and regret, are completely normal to experience, so it's okay and understandable to feel those things. However, you will not benefit from dwelling in feelings such as these. The past cannot be changed or revisited, so the only thing to do is accept it, forgive yourself, and move on.
Everybody makes mistakes; it's simply human nature. Think about all of the times that you have forgiven someone else for messing up. Ask yourself why you can't do the same for yourself.
Don't look back; look forward. Acknowledge your mistakes, yes, and try to make sure that you do not make the same mistakes in the future. Then, walk right on forward and forget the things that you feel remorse over.
Trust is key to all healthy relationships. Therefore, in order to have a healthy relationship with yourself, you need to be able to trust yourself to make the right choices in the future, regardless of what may have happened in the past.
Regardless of your age, you are still a growing and changing person, whether you be 18 or 81. Not only are you a growing and changing person, but you are also a good person, even if you feel as if you have done not-so-good things in the past.
Move on. You will feel better.