Not forgiving is like drinking rat poison and then waiting for the rat to die.

Anne Lamott (b. 1954) American novelist and non-fiction writer

Traveling Mercies, ch. 3 (1999)

On Facebook (31 Jan 2013) she further wrote:

When I first got sober in '86, I first heard someone say that harboring resentment is like drinking rat poison, and waiting for the rat to die. Resenting someone is about not forgiving them -- thinking that they have done something to you so damaging or disgusting that the are beyond the pale; so therefore you are choosing to be toxic for the rest of your life, rather than to work and pray for the healing. You are willing to go through life not metabolizing the rat poison, so that this person should know what a morally repellent person you believe them to be. But the most horrible thing is that half the time, they aren't even AWARE of what it is you think they did to you. So it's a complete waste of your precious bile. When I am willing to have clogged bile ducts, because of a person who hardly thinks of me, or has no idea that he behaved like a total asshat, then I'm the crazy one.

See also Fisher.