From experiences personal (several) and clinical (too numerous to count), I've seen a lot of forgiveness. Many failed attempts at forgiveness, I should say, and some satisfying ones that seem to last. The effective attempts tend to share four common elements:
The alphanumeric bullets are intentional. These are elements, not steps, as it's not a completely linear process. People need to go through points A-B-C in whatever order they deem necessary, even repeating them until satisfied. They may need to understand why or acquire safety before they let themselves feel. Or maybe they need to revisit the facts several times before they understand. But those three elements need to be achieved before letting go (point 4) is possible. A lot of people want to leapfrog the feelings and rush to letting go, that's a problem I often see. But bypassing the emotion doesn't allow a deeply satisfying process to take place.
I'll expand on this:
A. Express the emotion: Whatever the crime or injustice or violation, the forgiver needs to fully express how it made her feel. If the transgression elicits anger or sadness or hurt, those feelings need to be deeply felt and expressed. If it's possible to express it to the perpetrator, great. If not, a stand-in, empty chair, heartfelt letter or yelling in the car with the windows rolled down might suffice. Are you expunging all the feelings? Probably not, but enough to allow you to focus on the other areas.
B. Understand why: Our brain will continue to search for some explanation until it's satisfied. Maybe we won't agree with the rationale, but we need some schema that explains why the act took place. In some situations, even an acceptance of randomness can be a sufficient paradigm.
C. Rebuild safety: The forgiver needs to feel a reasonable amount of assurance the act won't recur. Whether it comes in the form of a sincere apology from the perpetrator, a stronger defense against future attacks or removal from that person's influence, safety needs to be re-acquired. To a reasonable amount, of course, because we are never 100% safe.
These three elements help us process the event. It's how I feel, how I understand what happened, how I know it won't happen again. On to the fourth:
4. Let go: This very difficult step is a decision. Letting go is making a promise to not hold a grudge. In the case of a relationship, it means one partner won't refer to that past transgression again: "I'm forgetful?!? Well, you forgot our anniversary once!" It's resolving to refrain from lording the transgression over the other in the future. When it comes to forgiveness, the victim holds all the power. I've even seen a smile creep over the face of someone who has been trespassed upon: "You screwed me over? That gives me a whole year of guilt-tripping." Letting go means surrendering this dominant role; a stepping down from the powerful position of victim to allow equality again. In addition, letting go is making a promise to yourself that you'll stop dwelling/replaying/ruminating/perseverating on the injustice. If letting go feels impossible, it's probably because A, B or C weren't sufficiently completed.
For example: I loan you my car and you return it with an empty gas tank, Arby's wrappers everywhere and an N'Sync bumper sticker. That's bordering on unforgivable, but I'm going to try:
I let myself feel the anger about the car and tell you about it (A). I say something like: "Dude! I'm livid! I hand you my car keys and this is what you do? I can't believe it! And who listens to N'Sync?"
Then I ask you why you did this (B). You tell me you were in a hurry and didn't have time to clean it out or get gas, but you meant to leave me $30 for gas and a car wash. And the bumper sticker was a bad joke. Lol? I don't like it, I don't agree with it, but it does help me make sense of the situation.
Now I need to rebuild safety (C), so I decide I won't lend you my car anymore or I ask you to promise not to do this again or I start charging you a $100 deposit for car borrowing. Something like that, something that helps me gain assurance that it won't recur. Then maybe I need to express anger again or set another boundary. If I've gone through these three elements; expressing feeling, gaining understanding and working toward feeling safe, then the last step is a little easier.
I make a decision to let go (4), so I won't hold this over your head in future conversations or conflicts. I'm also letting go of my rumination and perseveration about the issue. That's where the real benefits of forgiveness lie.
By the way, you're gassing up my car and taking off the bumper sticker, too. If it's possible/reasonable to make these reparations, it should be done. And when I find your Celine Dion cassette in my tape deck, it's mine to dispose of how I please.
Of course, many situations are more complex than this example. But in general, forgiving means returning to a place of equality. It promises health benefits, relational perks and emotional strength, as you know, but it also means relinquishing power. Maybe that's why it's so difficult for so many people.