Ever since Pastor Guy did a sermon on forgiveness, I have heard different people wrestling with the concept. The number one issue is that they aren't "ready" to forgive. At the risk of seeming very black and white, forgiveness isn't about our "readiness". It is about our obedience.
Nowhere in scripture does it tell us to forgive when we aren't mad or hurt anymore. However, in 2 Corinthians 2:11 God's word says to forgive so that we may not be outwitted by Satan. Clearly, unforgiveness opens us up to all kinds of bitterness, and has the potential to take the joy out of every other situation.
I have many occasions where I have wanted to withhold forgiveness, instead preferring to wallow in the injustice of whatever had taken place. I would wear these things as badges, giving me permission to feel sad, picked on, better than someone else. For me, it boiled down to feeling self-righteous.
Also, forgiveness does not depend on the size of the offense. I have had things happen where I may have been justified by the world for not forgiving, but being able to forgive those things has helped me grow. Weirdly, it is not the "big" stuff that I have a harder time forgiving, but the petty stuff.
On the other hand, let's say you have taken the high road. You have tried to forgive and forget, to move forward. You have made a conscious decision to leave the past in the past. But there is that nagging voice in your head, reminding you of what that person did. Every time something new comes up, the past comes flying in to stand along side it so it is no longer one offense, but a series of offenses. It happens to me all the time. When it does, make no mistake that that is not the Lord's voice. It is the voice of the enemy who would prefer to see us looking in the rearview mirror all the time, rehashing whatever got to us in the first place. If you "resist the devil, he will flee from you." James 4:7. I have this little self-talk that I do. I tell myself that I have forgiven those things and I ask God to help me not dwell on them. I can promise you that the things that used to flood my mind do so no longer. But it is a choice and a discipline. Kind of like exercise, but only my heart is getting into shape.
If you are struggling to forgive, remember that our Father forgives us not because we deserve it, but because He chooses to. Don't wait until you feel like someone deserves it, because forgiveness is to be given freely, unconditionally.
Finally one last "how can she say that?" thought, that is not mine but God's word:
"For if you forgive others their trespasses, your heavenly Father will also forgive you; but if you do not forigve others, neither will your Father forgive your trespasses." Matthew 6:14