I recently heard a statement that painted a vivid picture of what it can be like when we choose not to forgive. "When we make the decision not to forgive someone, it's like us drinking poison and expecting the one who offended us to die."
A long time ago, when I had just learnt to drive, I ran out of petrol on a lonely country road. I gratefully accepted some fuel from a nearby farmer. What he didn't tell me was that it was a mixture designed not for a car, but for a lawn mower. I got back home all right, but the next day the car behaved like a sick animal, coughing and spluttering.