Regret about mistakes, while not pointless, should be aimed at improvement rather than self-punishment. If you managed to make a mistake, then recognize what caused you to make that mistake. If it's something immutable or something you don't want to let go of, find other solutions. If it's easily changeable or desirable to change, do it. Be careful with what you consider immutable though, but if you do, then do not regret the immutable. Those "mistakes" simply exist, and are not inherently bad.