Don't hold on to the negatives in your life. Every mistake, every hurt inflicted by another...acknowledge them and let them go.
After something has happened its irreversible. The only option is to deal with cards you have in your hand right now. If you've made a mistake or screwed up, apologize and acknowledge your wrongdoing and move on. No matter what you do, chances are you can't take back what you did so mental flagellation serves no purpose other than to ease your guilty conscience...and it certainly doesn't help the situation.
And, if someone's hurt/betrayed you, acknowledge the hurt and anger but don't hold on to it. A lot of people just hang on to their resentment and continue to nurse their pain and hurt long after the fact. And, honestly, that does absolutely nothing! You end up just an emotional wreck because you're so committed to wallowing in your anger/hurt/resentment meanwhile the person that wronged you just goes on their merry way (unless you're the vengeful type).
Really, when push comes to shove...you just have to take what life hands you and move on. I'm not saying that you have to take it lying down, but its pointless to just hold onto things and wallow in misery. Life goes on...and so should you.