Forgiveness rarely feels like a single decision. It feels like a series of them, often on the same day, and often about something you thought you had finished with.
So do not be alarmed when resentment returns after you have forgiven someone. Its return is not proof that the forgiveness failed. It is evidence that you are a person with a memory, doing something difficult repeatedly. What matters is what you do next. You can follow the thought down the road it has travelled before, or you can hand the debt over again, briefly, without ceremony. He has been forgiven far more than he is being asked to release. That holds on the twentieth time as much as on the first.