One of my teachers had each one of us bring a clear plastic bag and a sack of potatoes. For every person we'd refused to forgive in our life experience, we were told to choose a potato, write on it the name and date, and put it in the plastic bag.
Some of our bags, as you can imagine, were quite heavy. We were then told to carry this bag with us everywhere for one week, putting it beside our bed at night, on the car seat when driving, next to our desk at work.
The hassle of lugging this around with us made it clear what a weight we were carrying spiritually, and how we had to pay attention to it all the time to not forget, and not keep leaving it in embarrassing places. Naturally, the condition of the potatoes deteriorated to a nasty slime. This was a great metaphor for the price we pay for keeping our pain and heavy negativity!
While too often we think of forgiveness as a gift to the other person, it clearly is a gift to ourselves!! So the next time you decide you can't forgive someone, ask yourself isn't your bag heavy enough?