For most, forgiveness is not something that happens automatically. Like many things in life, it's a process. An effective place to start is by taking a few minutes to weigh the benefits of forgiveness against the costs of holding on to a grudge. Next, in a calm and nonjudgmental manner as possible, review the offending situation. Look at it from all sides and not just yours in which you feel victimized. Was your reaction more than what the offense dictated? How has this situation and your reaction to it impacted your life? Does what happened and all its negativity have the right to occupy so much of your time and mental space?
Putting the off ending event in context with the rest of your life often reduces its importance and allows you to see how much of a waste of time it is to dwell on something that is in the past and is only standing in the way of your striving forward and getting on with being happy and participating in the more productive aspects of life. Define yourself not by your hurt but by how you are able to find compassion and understanding in the face of a perceived slight. To get even more out of your forgiveness, just don't tell yourself that you're going to let go, but let the other person also know how you feel.