Often, when we build up our defenses around the heart, we are trapping ourselves inside the walls with the pain we are trying to protect.
Forgiveness is the intentional practice of "giving up any hope for a better past", learning to dissolve the resentments towards ourselves and others that keep us bound to past experiences. We can learn to understand that harm is caused out of suffering, and we can learn to let people back into our hearts, without letting them back into our homes.