We all make mistakes. Sometimes they are very bad and cause ourselves and others much worry and suffering. We can be depressed and burdened with regret for many years, never forgiving ourselves, never forgetting our actions. Carrying on in this way is another big mistake, disturbing our lives in very unpleasant and painful ways.

It is not necessary. Worrying about something that has already happened is a complete waste of time if you just keep wishing that it hadn't happened. There is absolutely nothing you can do to change it.

"We ought not to look back unless it is to derive useful lessons from past errors, and for the purpose of profiting by dear-brought experience." - George Washington (1732-1779)

Perhaps there is some way to make amends and that should still be a consideration. If not then regard it as a learning experience. Review what has happened enough to draw conclusions about what to do, and what not do, in the future. But then forget the stupid or bad thing you did and start thinking about something else.

For every evil under the sun,
There is a remedy, or there is none;
If there be one, try to find it,
If there is none, never mind it.
- Mother Goose

Alright it isn't all that easy Mother Goose! These kinds of thoughts will keep coming back again and again unless there are other thoughts occupying your mind. Begin feeding your brain with encouraging and fun ideas especially when that 'thing' from the past returns. Be generous, kind and forgiving to others and yourself.

You no longer have yesterday; you only have today and tomorrows.