I like the expression that a resentment is like holding a burning hot coal in my bare hands with the intention of throwing it at someone. It's only hurting me, but something inside me doesn't feel it right away. It's like I'm wearing teflon gloves. Instead, I get perverse pleasure from rehashing what you did (or didn't do) to me. I enjoy dredging up evidence of your guilt--going all the way back to the beginning of our relationship if necessary--and presenting it over and over again to the judge and jury that live in my mind. The verdict is always the same in my courtroom. Guilty! I sentence you to go without my love and affection for as along as I can stand it.
The penalty phase might last days, even weeks. Since I burn with anger I am incapable of peace of mind and happiness. But just when I want to throw in the towel I think about your offense and I can't resist chewing on that bone for a while longer. Only when the pain is overwhelming am I willing to look for a way out.
Once I see that my desire to punish you is only punishing me, I can get started on the work of letting go of my resentment, not for your benefit, but for mine. First -- and the most fun -- is getting very clear about your guilt. I start writing and end up with a nice long list of all the stuff you've ever done that you shouldn't have: tube off the tooth paste, crappy birthday present, eating the last cookie, failing to return my call promptly in November, 1997, etc. I grade myself on the length of the list I compile. Once, during a resentment against my wife, I came up with three pages single-spaced, and we had only been married for five weeks! A+.
If I'm not looking for my part in a resentment, I won't find it. But again, if I'm in enough pain, as I review my list, I'll reluctantly try to locate the teensy weensy part I played. Usually my part is an expectation I had about some aspect of your behavior towards me. I realize that I had no right to expect you to behave like any normal, rational, loving, considerate human being would behave, but I don't tell you that. What I tell you is that I'm sorry I tried to make you responsible for my feelings. And that I accept you just as you are.