The only way to avoid bitterness is to practice forgiveness.
How can we do this?
First, we have to realize WHAT forgiveness is. Forgiveness is granted before it is felt. Luke 17:3-6. Forgiveness is primarily a promise to…not bring the wrong up with the person; not to bring it up with others; and not bring it up in your own thoughts. It is a promise not to dwell on the hurt or wish ill on the other. These are under the control of your will. You are not able to keep thought from occurring to you, but you don’t have to entertain it.
Second, we have to realize HOW forgiveness is possible: only because you see and feel the reality of God’s massive and costly forgiveness of us through Christ. (See Matthew 18:21-35). Only the knowledge of our debt to God can put into perspective someone else’s debt to us. The forgiveness of Christ gives us the emotional humility to forgive (who am I to withhold forgiveness when I am such a sinner?) and emotional resources to forgive (what has this person really robbed me of when I have so much in Christ?).