Bob Marley said it best...
"The truth is, everyone is going to hurt you. You just got to find the ones worth suffering for."
It's hard when someone lets you down to the capacity of a plane crash. I was asked the other day by a friend "how have you learned to let go?" It's true that everyone at some point of your life will let you down. There is no stopping it, that's just the way life goes. Even the people nearest and dearest to you will do this. Your friends, family, mother, father, brother, sister, best friend, spouse and some day your own children.
We all have our faults. Every single one of us. No one is perfect, and why should we be? Whether you are aware of it or not you too have hurt someone to this extent. You can't forget your own transgressions. So the idea of forgiveness is to appreciate that... recognizing that everyone is human, and while it does hurt like a bullet to your gut the pain will eventually subside. And when it does you have to decide if that person is worth the pain and suffering.
Forgiveness isn't about what someone did, its about the pain behind it and are they worth it.