A teacher once told each of her students to bring a clear plastic bag and a sack of potatoes to school. They were instructed to call to mind every person they had a grudge against. For every person they refused to forgive, they chose a potato, wrote on it the name and date, and put it in the plastic bag. They were told to carry this bag with them everywhere, putting it beside their bed at night, on the car seat when driving, on their lap when riding, next to their desk during classes. Some bags became quite heavy. Lugging this around, paying attention to it all the time, and remembering not to leave it in embarrassing places was a hassle.
Over time the potatoes became moldy, smelly, and began to sprout "eyes."
Often we think of forgiveness as a gift to the other person, but it clearly is a gift to ourselves.