Let all bitterness and wrath and anger and clamor and slander be put away from you, along with all malice.  Be kind to one another, tenderhearted, forgiving one another, as God in Christ forgave you. (ESV)
When I think of the word "bitter," I think of food. Maybe this says something about my unhealthy eating habits - or maybe it's just a good analogy. We'll go with the latter. Some food is sweet, like honey or watermelon. Some food is savory, like bacon or spaghetti. Some food is neither sweet, nor savory; some food is just bitter. Try eating a 99% dark chocolate bar and you'll know what I mean. While it might be enjoyed in the proper setting, bitter foods usually leave us with a bad taste in our mouths. That bad taste is unsettling to us, and if we were wise, we would spit it out immediately.
Unfortunately, when it comes to bitterness of spirit, we are slow to spit it out. We tend to cling to our bitterness, feeding on it's sour flavor to appease our own pitiful consciences. Why do we do this? Well, I think we answer that question with another: where does bitterness come from? I recently read Jim Wilson's little handbook on letting go of bitterness, and he makes the argument that bitterness is the opposite of guilt. When we sin, we feel guilty and condemned. When others sin against us, we feel bitterness. If we understand bitterness to be what we experience when we are wronged, it makes sense why we are slow to let it go. We want the other person to feel guilt as long as possible, because we too, are sinful. But like sucking on a persimmon, it does more harm than good.
What does the Bible say? The Paul writes to the church in Ephesus a command to put it away. And not only bitterness, but all her companions as well, which are wrath, anger, clamor, slander, and malice. The longer we hold on to bitterness, the more bitter we become and the harder it is to hide. We are never satisfied, always disgruntled, and constantly giving off a "holier than thou" vibe. Our bitterness spreads to others, causing division and strife among the people we love. It's amazing how we have the capability to hold on to bitter wrongdoings for years, decades, or even a lifetime, resulting in our immanent loneliness. This causes us to become physically ill, and no doctor can fix the chronic pains we will face. There is but one solution: forgiveness.
We become free from bitterness by spitting it out. We get on our knees, we confess our sin of bitterness, and we repent, asking for God's grace to live with contentment in the forgiveness of Christ. He has forgiven our sins, so we must forgive those who have trespassed against us. This means when we get up off of our knees, we truly "let it go." If we find that our mind continues to go back to what "that awful person said to us," than we haven't let go of bitterness. We haven't seen our bitterness as the wicked and gross sin that it is, which blasphemes the very Gospel of Christ. This is easy to see in others, but terribly difficult to see in ourselves.
If we will humbly begin the process of putting away bitterness, we will soon find that we have to dig beyond the surface. We have to dig down the root of the bitter tree that has been planted, and cut it off at the soil level. If we all made this part of our regular Christian living, the church would be a much happier family, and the whole of evangelicalism would not be so depressing. Christ forgave us; let's forgive one another.