Word for the act of being unwilling to forgive?

I'm developing a novel, and I'm having a hard time describing a major characteristic of the main character. We'll call him Dan for the moment.

There was another person (we'll call him/her "Francis" for now), who did something bad to Dan. Dan cannot forgive Francis for this action. Time has gone by, and Francis has apologized for the action, and is ready to make up. Dan however, remains stubbornly holding onto his ___. That's the word I need. 'Unforgivingness'.

It's more than simply not forgiving someone. It's more active than that; more like staying angry at them when you know it's over. Essentially the opposite of 'forgiveness'. Forgiveness means the 'act of forgiving', so the word I'm looking for would mean the 'act of consciously refusing to forgive someone'.

Example sentence:

"You drove them away, through your own ___ (unwillingness to forgive them)."

Is there a single word for this I can use?

The word that comes to my mind for literally referring to being incapable to forgive is implacability, the quality of being impossible to placate or unable to be appeased.

But in your first sentence, I'd go with resentment and in the second, implacability:

Dan however, remains stubbornly holding onto his resentment.

You drove them away, through your own implacability.

Not a property of Dan, so not exactly what you are asking for, but you might be able to use "grudge":

...stubbornly holding onto his grudge.

Why not simply "unforgiving"?

As in, "Dan, however, remains unforgiving."