Indulging bitterness is one of the stupidest things we ever do. How stupid is it? Think of it this way: Imagine someone stabs us in the arm with a knife. After he flees, we take out the knife, and then begin impulsively to stab ourselves again and again, with our own hand, out of our own choice. We do this again and again, day after day, week after week. It’s just about the stupidest thing we could ever do. But that’s exactly what bitterness is. Someone hurts us, lies to us, gossips about us, fails to support us. It hurts. It hurts badly. It’s like they stabbed our heart with a sharp knife. But rather than turning to God, we keep thinking about the wrong. We play it in our mind like a video, playing it over and over. We stab ourselves with a sharp memory of their wrong, again and again. This is why the Bible reminds us to get rid of all bitterness and malice. God wants to spare us the pain and trouble. The best way to get over those bitter memories is to rejoice over and over in what God has done for us through Christ, and then to ask Him to fill us with grace and love so we can respond to those who wronged us.

Ken Sande, from Focus on the Family

Here’s the link to the video this paraphrased quote came from:

Section taken between  8:17 – 10:26.