A teacher once told each of her students to bring a clear plastic bag and a sack of potatoes to school. For every person they refuse to forgive in their life's experience, they chose a potato, wrote on it the name and date, and put it in the plastic bag. Some of their bags were quite heavy. They were then told to carry this bag with them everywhere for one week, putting it beside their bed at night, on the car seat when driving, next to their desk at work....
The hassle of lugging this around with them made it clear what a weight they were carrying spiritually, and how they had to pay attention to it all the time to not forget and keep leaving it in embarrassing places. Naturally, the condition of the potatoes deteriorated to a nasty, smelly slime. This was a great metaphor for the price we pay for keeping our pain and heavy negativity.
Too often we think of forgiveness as a gift to the other person, and it clearly is for ourselves!