The (im)possibility of forgiveness

As I come closer to completing my second PhD which focuses on concepts (and processes) of forgiveness and reconciliation I have been thinking a great deal about the complexity of true forgiveness.

I often hear people saying "I cannot forgive that him (or her), what they did to me was simply too bad".  Indeed, forgiveness is difficult.  Is it ultimately about gaining my own freedom?  Or is it about giving freedom as a gift to the 'other'?  Or, is it an interplay of both of those?

I found this quote from Jacques Derrida on forgiveness quite challenging in the possibility, and impossibly, of forgiveness - I like to phrase it as the (im)possibility of forgiveness.

Forgiveness only becomes possible from the moment it appears impossible.

...

If there is something to forgive, it would be what in religious language is called mortal sin, the worst, the unforgivable crime or harm. From which comes the aporia which can be described in its dry and implacable formality, without mercy: forgiveness forgives only the unforgivable. One cannot, or should not, forgive; there is only forgiveness, if there is any, where there is the unforgivable. That is to say that forgiveness must announce itself as impossibility itself. It can only be possible in doing the impossible.

- Jacques Derrida

Have you ever forgiven someone for something that seems unforgivable?  How was it possible?  What helped you to do it?  Did you follow a process?