It seems to me that on a mental level it is the injuries that we do not forgive that fester inside us. If we can forgive, then the injury becomes less important, we do not experience the anger and thirst for revenge, it ceases to live inside our minds.
I had an experience while meditating some years ago. It was about a guardian spirit who lived in my aura. I was visited by two travelling spirits, one of whom took a long spear and used it to slice into my nose. The guardian spirit who was there took this very seriously, he felt umbrage, anger, but he suppressed it. The other traveller then said, “if you had forgiven the injury it would have been better for you.” The guardian spirit, once noble, was darkened by his feelings of anger and spite and revenge.
It struck me just recently that something like this carries a message. All through my life i have felt a great attachment to justice and fairness, and when people treated me unfairly I used to get furiously angry. I can recall a few instances, sleepless nights from being angry. But in fact that anger wasn’t to anyone’s benefit, and I knew better than to seek revenge.
Injustice, being treated unfairly, that can leave a mark, vengeful feelings in the mind. It seems clear to me now that it is better to forgive, to realise that those who damage you are also just carrying out the patterns of behaviour that were given to them. That way, you can let go of whatever negative feelings you were carrying in the mind.