Who doesn't make mistakes? No one. We've all done something "stupid" in our lives and it's so common. The past is in the past and you can't do anything to change it, don't look at it besides if you need to figure out how to change the future by effecting the present. Also, regretting is only a waste of time, it does nothing. If it's related to people then talking to them is usually the best thing to do
