No wonder I find the religious approach so difficult.
So what do we say about an angry, wrathful god? Ah, but this isn’t that thread. 
A more Eastern approach would be to say that Guilt and Shame are the sins here.
But regardless, the key is to recognize that you’re not doing yourself any favors, and to change your way of thinking. Religion may well help in allowing ourselves the idea that we don’t have to have all the answers. Surrendering the shame of Powerlessness.
What’s always worked for me is to destroy my enemies utterly. Crush their bodies, burn down their houses, salt the scorched earth. If they’re dead and gone, you don’t have to think about them any more.
I have nothing useful to add; I only wanted to thank RipleyNow for starting this thread. I tend to suffer from this too, never would it had came to mind to ask people’s opinions on it.
Ii would also like to thank the other posters for their advices. 
I’ve personally found that meditation and mindfullness practice works wonders. I’ve gotten my guidance from buddhist sources, but I’m sure other meditative practices can work just as well.
Someone earlier referred to anger as a “sin”… but I think the word “sin” is too judgemental and will lead to further self blame. I prefer thinking of it as merely being “unskillfull” and direct my mental energies toward more “skillfull” things.
“Life is lumpy. It’s full of lumps. Lumps in your oatmeal, lumps in your throat, and lumps in your breast. Learn to tell the difference and act accordingly.”
Someone much smarter than me, who may have been quoting someone smarter still, said that to me once. It’s a little pithy, sure, but it makes a lot of sense.
When I get angry, I’ve actually trained myself to take a mental step back and classify the situation into “lumps.” Most frustrating things really are just “lumps in your oatmeal” and just realizing how trivial they will turn out to be in the long-term helps me get over my anger immediately.
I discovered the lesson everyone is talking about last year. All my life, I had heard people say, “Just let it go,” “Don’t let it bother you,” and, “Get over it.” I always thought that was simplifying the issue way too much. After all, the event(s) would torment anyone, don’t they even care? What kind of advice is that.
Last year, I was fired. I celebrated with cake because I wanted to get out of that job so much. I had been applying at various other companies because I was being bullied at work. I reported every incident to my boss and he did nothing…except report to my bully that I was “telling on him.” It was middle school all over again.
Upon leaving that hostile environment, I felt extremely liberated. I was noticing how fresh and cool the air was, how good food tasted. Everything felt better and brighter, which caught me off guard because it wasn’t logical. Upon realizing that there was such a dramatic change in my perception of the world around me, I finally “got” that I could also be tormented internally if I let the bully continue to dominate my thoughts.
I realized that I could never solve it, because it was an illogical problem. The corrupted coworker and the corrupted boss happened to both have problems which reinforced each other. Why would a boss fire me, instead of the racist, sexist, violent-tempered half-wit? It was unsolvable. There is no reason. But the physical sensation of long-forgotten freedom made me aware that I had been doing the same thing with myself emotionally. I didn’t want to weaken myself to the point where I had to become a bully.
A saying that I coined for myself was “The cycle of evil dies on me.” I still walk around feeling like my purpose is to be the place where evil/anger goes to die. That keeps me aware that I will have to bear a lot of burdens in life, but the “act” of letting it dissipate and lose momentum on me is an effective way to terminate a destructive cycle that has probably tormented one person to the next for a very long time. I am the place where that ends, and this makes me happy and peaceful.
This is one of the true lessons of life. We put ourselves through such torment! We stay in bad situations, in work, in personal relationships, in marriages, because we’re afraid of change. But the truth is very often that the death we fear, the death of that relationship, is not true death, but LIBERATION!
I got fired at the end of December. I knew it was coming, I didn’t fear it, because I had been through it before. My job environment was so toxic, the people so corrupt and evil, that I walked out with a smile on my face!
Honestly folks. Look at your relationships, whether they be personal, or a job, or a commitment. If you wake every day dreading that day, if you fear all interaction with that person, if you hate yourself for putting up with it…
Let It Go.
I guarantee you this. Staying is much worse than leaving. Oh, I can’t guarantee that it will be painless. But a short period of “surgery” or a lifetime of torment, which do you choose?
LET IT GO.
Thanks everyone.
I certainly seem to have been less susceptible to dwelling on these past wrongs in the last few days. I still haven’t quite cracked it, but thank you everyone for the insight.
This is something I’ve been working on.
When I get stuck on something, I try to bring myself back to NOW. If there is something I can do about it now, I do it. If there isn’t, I realize that the problem is not the incident, but my reaction to it. So I remind myself that it is over and not affecting me, and usually that calms my anger about it.
Sometimes, though I am not religious, I’ll think back to phrases in the bible about forgivness, etc.
Play a lot of GTA IV.
I’m only half kidding, I think violent video games can function as an outlet.
Since the OP asked HOW to do it, not WHY, that’s what I’ll answer.
This method may be a bit silly, so feel free to get as self-conscious as you like. It’s all inside, so no one’s going to laugh at you.
Step 1: Remember the grievance. Yup, think about. Remember everything you can.
Step 2: Remember sometimes you’ve done something similar. Or, if you haven’t remember sometime you did something you are ashamed of. It’s OK if the person who harmed you isn’t sorry.
Step 3: Say to yourself: “I forgive that. I let that go. It doesn’t have any power over me.” You don’t have to hold on to it. It’s a choice. You can go live your life without worrying.
Step 4: Let go and… do something else. Think about something else.
Accept that some things take some time to get over, but the sooner you can do this, the better.
From The Four Agreements :
#2 and #3 are imperative in your situation.