Think about it. Can we change our past? No, of course not. What we can do is change the attribution we assign to our past - in other words, we can change the meaning we make of the past. As another friend of mine (Gary Levine) says, "Our past is not our future."
No amount of anxiety, fretting, anger, etc. is going to do a thing to change mistakes we have made or unfortunate situations that we have experienced; but if we use our energy to move forward in our pursuit of happiness (engaging in those activities that bring satisfaction and meaning) we will overcome, have a greater sense of freedom and will do a better job of fogiving ourselves and others.