There is a part of you that simply feels things, but has no capacity at all for language, or any other abstract ideas such as time, grace, love, truth, or justice. That part of you, the seat of your emotions in the brain, the part of you that just feels, is called the limbic system.
That explains why, when someone hurts you and then apologizes, you may genuinely forgive them and yet the pain may linger for quite some time. Your limbic brain doesn’t understand right and wrong, or apologies — all abstract ideas — it just feels. So it takes time for the feelings to subside. While that time passes, most people (especially religious ones), beat themselves up for not genuinely forgiving.
The takeaway here: There’s literally nothing you can do about it. It just takes time.
There is another part of your brain (this is called the “triune brain” theory, by the way), even more primitive, called the lizard brain. The lizard brain controls more or less automatic things like your heartbeat, digestion, swallowing, etc. This is located in the brain stem. The “fight or flight” mechanism is located here.
The stem is the most basic part of the brain. It is wrapped in the limbic brain, and then the neocortex (the part that evolved most recently) is on the outside.
Why does any of this matter?