Resentment is an extremely bitter diet, and eventually poisonous. I have no desire to make my own toxins. -Neil Kinnick
Getting hurt by someone you care about is truly one of the worst pains in life. Have you ever gone through a hurt that you just couldn't let go? What does it feel like to hold a grudge? Perhaps even the thought of person that hurt you burns into your brain like battery acid, leaving a deep groove that seems impossible to climb out of. Perhaps that acid seeps into other parts of the your body as well, causing all sorts of inexplicable aches and pains. Interestingly, a recent survey of almost 10,000 Americans found that those who carry a grudge have higher rates of stomach ulcers, arthritis, back problems, headaches and pain. 
This survey also confirmed something that we have long suspected: hard-heartedness is very bad for your heart. The survey participants who reported less forgiveness were more likely to have experienced heart disease, heart attacks, and high blood pressure. For these individuals, the following quote by Malachy McCourt is sadly appropriate: "Resentment is like taking poison and waiting for the other person to die." On the other hand, another study on cardiac patients reported that higher levels of forgiveness results in lower levels of anxiety, depression, stress, and bad cholesterol, which all decrease the risk of future cardiovascular events. 
"Gee, thanks," you might be saying. "I already figured out that an unforgiving heart is bad for my health. Can you tell me what to do about it?!"
Dr. Dick Tibbits, psychologist and Chief People Officer at Florida Hospital, has written a magnificent book on this subject: Forgive to Live: How Forgiveness Can Save Your Life
Here are the 10 principles of forgiveness, as outlined in his book:
- Accept that life is not fair and that not everyone plays by the same set of rules. Although this can be a tough pill to swallow, remember that forgiveness does not necessarily mean that you condone the behavior. Rather, forgiveness is deliberately choosing to overlook the behavior -- not for the other person's benefit, but for your own. Forgiveness sets you free from all the negative emotions that can quite literally eat away at you mentally and physically.
- Stop blaming others for your circumstances. What happened in the past is, simply put, in the past. Where you are now is your business. Don't allow your offender's actions in the past to steer your present life.
- Understand that you can not change the other person; you can only change yourself. Don't focus on the other person. Instead, find and capitalize on the opportunities for personal growth this situation has given you.
- Acknowledge the anger and hurt that the past event is causing you. Anger can be a positive emotion if it motivates you to find your way out of a negative situation. Learn how to control it and express it in a constructive way.
- Re-frame your "grievance story" by placing it into a broader perspective. Think of individuals like Corrie ten Boom and Victor Frankl, survivors of Nazi concentration camps, who channeled their stories into personal testimonies that continue to help millions to learn how to forgive.
- Recognize that only you can make the choice to forgive.
- Empathize with your offender. "To err is..." That's right, "human." Remember that we have all, at one time or another, needed the forgiveness of others.
- Intentionally move from discontent toward contentment. Even in the best of circumstances, there will always be something to be unhappy about. It is equally true that there will always -- ALWAYS -- be reasons to be grateful and content. The choice is up to you.
- Understand that forgiveness will take time and can not be rushed. There will still be times when that person or event will cross your mind. With those memories may also come some pain, and that is perfectly OK. Give yourself time to heal.
- Take responsibility for your life and your future. Instead of throwing your energy into the black hole of bitterness, invest it in living the rest of your life in a positive, fulfilling way.
Remember that a life well-lived is the best (and healthiest) form of "revenge." And when you can think of your offender and sincerely wish him/her a good life as well, you will know that you have achieved forgiveness.  Messias E, Saini A, Sinato P, Welch S (2010) Bearing grudges and physical health: relationship to smoking, cardiovascular health and ulcers. Soc Psychiatry Psychiatr Epidemiol 45(2):183-7.  Friedberg JP, Suchday S, Srinivas VS (2009) Relationship between forgiveness and psychological and physiological indices in cardiac patients. Int J Behav Med 16(3):205-11. (2006) Published by Integrity Publishers, a division of Integrity Media, Inc, Franklin, TN