One of my teachers had each one of us bring a clear plastic bag and a sack of potatoes to class. For every person we'd refuse to forgive in our life, 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 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!
Too often we think of forgiveness as a gift to the other person, and while that's true, it clearly is also a gift for ourselves! Carrying around animosity for others adversely affects our own lives.
The Prophet sallallahu alaihi wa sallam advised that we should enter every new day and night in such a state that we do not harbour hatred or malice for any person. He said that this was his Sunnah (way); that whoever loved his Sunnah loved him, and whoever loved him would be with him in Paradise.