has nothing whatsoever to do with how wrong someone else was; no matter how evil, cruel, narcissistic or unrepentant they are, when you forgive a person, you break the unhealthy bonds between you and your abuser-victim relationship, and you redefine yourself as an independent victor in your own life.
Forgiveness is not about them, it is about YOU. Even if what they did to you seems unforgivable, an act of forgiveness, on your part, sets YOU free.
Otherwise, they are always with you, you will be reliving the situation over and over again, they own you. In order to get out from under that, and get your power back, you forgive them. This is no way negates or minimizes what they did to you, you are not excusing the act, you are allowing yourself to get on with your life, without the burden.
You are here for a reason, this situation took place for a reason, you are not given more than you can handle, and the proof is in the ability to forgive. If you choose not to, you will live with anger, resentment or revenge and continue to be a victim of this person.
In order for you to have any kind of life, you want to move out of victim mode and back into your power. You are here on this earth and it is your choice how you want to live. This is an opportunity to have a complete life, filled with love and joy or be dragged down by the anguish of your story, never forgotten. It is not easy to forgive when you have been so hurt, but you owe it to yourself not to continue to be hurt. Don't let this injustice define you, redefine yourself as the victor in your own life.