The brain is the record of the past. Every single person, every single object, every single experience, every single place is mapped neurologically in your brain. You have a neurological network for your partner, pet, co-worker, phone, etc.
Something happened in the past that you can not forget? Let me tell you what's happening in your brain.
Think about frozen circuits in your brain. The more profound the insult you experience, the stronger the emotion that is associated with it. The more the brain freezes circuitry—a hard-wired map of the event in your brain.
The more you think about the experience and remember the event, the more you fire and wire the circuits in your brain. The more you produce, the emotion that is associated with the event.
After a certain time, it becomes so hard-wired that holding a grudge is not a conscious choice. It happens automatically, and you don't even realise that you see everybody through that lens.
Now, everybody is a betrayer; everybody can't be trusted. Everybody is wearing the mask of the past person. Anything in your life that becomes remotely close to it based on your limited perception triggers the emotion, and you start acting like the event is happening again.
This means that, as a creator of your own reality, you are holding yourself and them hostage because of that experience.
You have an energetic bond with that person. You share the same energy. You use each other to affirm whatever the energetic agreement is.
When you forgive someone, you change your emotional state, which means you break the energetic bond.
The less emotion you feel for that event, the more you call your energy back to you, and the less you pay attention to the event.
To forgive is to take your attention off someone.
And don't be surprised if that person shows up completely changed, transformed, asking for forgiveness even if it is years later because you freed not only yourself but them, too. This is the science of forgiveness.