I think so many of us are too hard on ourselves for what we didn't accomplish or what we should have done. The first step is to forgive yourself for all the things you didn't do that you should have and all the things that you did do that you shouldn't have. Get rid of the guilt. Negative feelings don't do you much good. The way to deal with them is to forgive yourself and forgive others.
Forgiveness is a tricky term. It does not only mean that you apologize, although regretting what you did is part of it. You may want to make amends if you can, but there are some circumstances where there is nothing more you can do. Even when you cannot mend fences with others, you need to tell yourself: "Yes, I did it and it would have been better if I hadn't, but now I want to forgive myself for having done that negative deed."
Forgiveness helps you come to terms with the past. I've learned how to forgive myself, and this has helped me no longer feel deep regrets or sadness about my past.
There's a difference between using your past and wallowing in it. Say I had an experience with a nasty person and I got nasty back, but I don't want to be that way anymore. I can use that experience to work out a different response whenever someone is not so pleasant to me. If I don't like my reaction, I can change my response.
You can review your past, benefit from your successes, and learn from your mistakes without judging yourself. This is an excellent time to do a life review, to make amends, identify and let go of regrets, come to terms with unresolved relationships, and tie up loose ends.