I'm not sure. If someone has deliberately done something to me that is "unforgivable" I distance myself from them and really never think about it again. If they contact me sometime later, I will talk with them, but I won't let them get close enough to do anything to me. If it's just some small thing, like forgetting to meet me or call me, there's nothing to forgive. I totally understand that people are not perfect. Being genuinely sorry goes a long way with me.
It's pretty easy for me. I have always had a forgiving heart. I've had a lot of really bad things happen in my life and there is not one person that I haven't forgiven. I don't want to carry that type of anger or stress on my shoulders. Much easier to forgive. That doesn't mean I allow people to take advantage of me.
Fool me once, shame on you; fool me twice, shame on me.