"Promise Yourself to forget the mistakes of the past and press on to the greater achievements of the future." (excerpt from "Promises to Yourself", also known as The Optimist's Creed by Christian D. Larson).
As obvious as it seems, sometimes it is hard to let go of past mistakes, whether they're our own shortcomings or someone else's. We tend to dwell on the negative things and get caught up in hypotheticals; 'what if's and 'if only's and spend so much energy assigning blame and picking things apart that we waste valuable time that should be used to build a better future with the newfound knowledge we have gained from the experience.
What's done is done; focusing on the negative doesn't change that fact. Forgive whoever needs forgiving, and remember that person may very well be you. Let it go and focus your efforts on the road ahead, with the positive idea that you are a better, wiser person with more life experience under your belt and a greater formula for success!