What has been the most difficult thing you have had to forgive?
The most difficult thing I have had to forgive is betrayal. I initially thought of being cheated on or having a friend stab me in the back. I realized that these both go back to betrayal. When you are in a committed relationship and your partner cheats on you, they have betrayed your trust. When you have a best friend and she becomes romantically involved with you partner that is betrayal of your friendship.
I have been betrayed many times by many people. I have come to accept that part of life is trusting even if there is a significant chance that your trust will be betrayed. A part of what makes trusting so hard is the possibility that you will regret having trusted someone. But it shouldn't keep you from hoping for the best and being trustworthy yourself. It took me quite a while to forgive those who have betrayed me, but I find that it is necessary for me.
Harboring anger and resentment toward someone that has betrayed you only hurts you. Most of the time the other person is out living their life carefree while you are stuck. Also forgiving is helpful for those that you will become involved with after the betrayal. You won't ever find a trustworthy person by trusting no one. You have to risk it daily to find those worthy people.