I used to get quite angry about past wrongs until I developed better analytical skills and was able to examine why the person may have reacted the way they did, my role in that, and why, exactly, I was upset.
I discovered that most situations really came down to me being misunderstood, and I was upset because the other person showed an inability to empathize with me and mischaracterized me and my intentions, which in these particular situations, were usually genuinely good intentions.
I over came most of my anger by realizing that it was a short coming of others that they themselves weren't perfect, may not have had the capacity at the time to interpret and respond to the situation more appropriately, and had to be forgiven for that because it was unreasonable to hold them to a standard which I myself didn't meet. I would subsequently think of more appropriate ways they could have responded to the situation and reasons they might not have.
Occasionally I would come to realize that the person was stressed and their anger had nothing to do with me personally, or maybe the person really was a jerk, or maybe their actions were justified and I owed them an apology.