You can save them in a "mental file," much like a valuable book. This teaches you not to repeat the same mistake twice. Remember that the past is filled with accomplishments, too. If you can read these words, you must be accomplishing quite a bit. Be fair with yourself and try not to be so critical. Let go of guilt and forgive yourself for being human. If you start to sink into deep dark past regrets, practice pranayama; breath awareness is the key to bringing you into the present.