How Do You Deal With Guilt?

Years back, in a heat of the moment, I said some real nasty things to my ex which I so much regretted and since, each time I think about it, I inevitably feel an awful pinch of guilt. It's like an apology would never be enough to make up for that kind of damage. We have each moved on. But even my ex could forgive me, I could not forgive myself. How do you deal with such a guilt?