There is only one path to freedom from bitterness of heart and that is forgiveness. Forgiveness is an all or nothing proposition, there is no middle ground here. No "sort of" forgiveness will do.
Forgiveness is an act of the will and motivated by a heart that understands how much you have been forgiven by God and how unworthy you were to be forgiven.
God commands us to forgive one another, even of the deepest hurts. Jesus said our forgiveness is to be without limitations (Matt. 18:22-23). This means that you must forgive completely, holding nothing back. This is quite impossible for you to do alone, and thank God you don't have to! The Holy Spirit will assist you in applying this command to your heart and life so there is no excuse of "I can't" because your "can't" is a won't!
I routinely hear several reasons as to why people can't forgive. One of them is "I can't forgive until I forget." Nursing the hurt this way will not heal you. The longer you continue to meditate on the hurt (in the name of not forgetting) the deeper bitterness grows.
Another common rational is "time will heal the pain." This isn't true. When the hurt is unforgiven bitterness grows because the pain is cherished and nursed along.
I have had women tell me they will forgive when the other person says they are sorry. This can be abused, and this is not the model we have in Christ.
Be on your guard! If your brother sins, rebuke him; and if he repents, forgive him. “And if he sins against you seven times a day, and returns to you seven times, saying, ‘I repent,’ forgive him. Luke 17:3-4
Yes, the offender also has an obligation to repent, but this does not remove the latter’s obligation to do all in his power to affect reconciliation.
Dr. Jay Adams says, “Forgiveness is a lifting of the charge of guilt from another, a formal declaration of that fact and a promise (made and kept) never to remember the wrong against the person in the future."[i]
When you forgive you commit not to dwell on the incident mentally any more (Phil. 4:8). You determine to bury it in the depths of the sea as God did. You also
commit to not bring the offense up to the person as a weapon (Matthew 6:12) in any future disagreement.
Forgiveness is a promise to not bring it up to others (Ephesians 4:29). When you forgive the offense is never brought up again in any hurtful way. This means you don't tell others about how the person hurt you. You don't share the information with anyone. To bring it up to someone is a clear signal you really have not forgiven and are possibly desiring revenge due to bitterness.
Theology of Christian Counseling.