What is unforgivable?

Is there such a thing as an unforgivable act?

If we accept that people have regrets, that people change, that people let you down, that we let people down, that everyone has the very best intentions (don't we all have all the best intentions?)

Then what is unforgiveable?

I do not believe in any kind of God.
I do not believe that forgiveness is my duty,
No-one demands this of me, that which I cannot give,
So why do I torment myself so?

And, he is out of my life, a friendship of ten years,
Lost in bitterness and tears
I can only forgive him inside...And I can't.

Does he forgive me?

We never said goodbye.