Luke 17:4 And if he trespass against thee seven times in a day, and seven times in a day turn again to thee, saying, I repent; thou shalt forgive him.
The prior is a very interesting verse to me, in that it is essentually saying, if someone asks for forgiveness, we are to forgive them, so if they don't ask, we we are not bound to forgive them, as many seem to think. Even God expects us to ask him for forgiveness.
That is not to say we cannot forgive those that don't ask, but for certian terrible things, like what happened to Holy999, Jesus understood how hard it can be to just forgive, knowing that the offending party may not even care about what they did. For me, if someone sincerity asks for forgiveness, I just melt away, any usually find it very easy to forgive, but otherwise, not so much.
I know there would be some argument about this, and many will say that we just need to forgive regardless, but remeber, it was Jesus who made the rule, and he made it for a very good reason.
Point being, if the person who did this hasn't asked for forgiveness, don't worry about not being able to let it go. You may be able to forgive them in time, or mabe not, but as long as they havn't asked, you are under no obligation.