Like Anna said, forgiveness is something that I have been learning in stages of awareness. Resentment is something I've really struggled with over the past few years, with two particularly strong grudges.
Others have already said this, but the first stage for me was realizing that my resentments hurt my own soul more than they've ever hurt the other person, and that letting go was a gift to myself. That awareness allowed me to dismiss a lot of little irritations, but it didn't tackle the bigger stuff.
The second stage seemed to be realizing that a lot of the people who really irritated me were sick themselves. That doesn't absolve them of their responsibility, but through awareness of certain personality disorders or taking stock of someone's background, I was able to develop an empathy towards them that was freeing to me. A good example is that when I looked at my mother's home life when she was growing up, I realized she entered adulthood with a really crappy parenting model and poor self-esteem. She really was doing the best she knew how to do, and that's all I can ask of her.
But after the above two stages, I still had two persistant, acidic grudges that I have struggled to deal with. And I think I have finally made a breakthrough over the past three weeks as I've learned more about boundaries. The more I've read--on this site and through a couple of books--the more I've realized that the reason I have not been able to release these two grudges is that I've continued to feel very vulnerable to abuse by these folks and others like them. By re-examining what went wrong in these relationships in light of what I've learned about boundaries, I've realized that I left myself open to their manipulation or abuse by not setting healthy limits on the relationships and not having the self-confidence to stand up for myself when I was being abused and manipulated. Like Eleanor Roosevelt said, "No one can make you feel inferior without your consent." And I had unwittingly given these folks consent to emotionally beat me up.
This new awareness of how my lack of boundaries contributed to a lot of hurt doesn't mean that I've simply shifted the blame to myself. Instead, it has been really empowering! As I've started mentally retracting my consent to inferiority, it has been liberating to realize I don't have to go through that kind of abuse anymore. I've started using those two experiences as case studies in how to strengthen myself for dealing with difficult people in the future--how to not put myself in a subservient position in any kind of relationship. I'm not expected to do so, and I shouldn't do so, so it's okay not to do so. Nice, huh? As I've become less vulnerable to this kind of threat, I've started to feel peace toward these two people.