Bitterness and unforgiveness are unforgivable sins in believers' life. You have to get rid of them. Bitterness is the gate through which the darkness and diseases can enter your life. If you have bitterness in your heart, you may a target of many attacks of demons like:
unbelief, doubt, fear, revenge, anger,
spiritual blindness, lies, deception,
defeat, depression, burnout,
abuse, slavery, anxiety,
illnesses, pains, accidents, weaknesses etc.
The deliverance from bitterness is indeed a great breakthrough in your life. Let past things leave from your soul. Confess your bitterness, renounce it and receive forgiveness and blessing of Jesus in your life. When you do this, Jesus pours a new anointing and healing into your life.
In this teaching, Jesus expressed that God the Father's forgiveness is based on His extreme loving compassion towards man.
Now when his fellow servants saw what had happened, they became very sorrowful and went and told their master all that had happened. Then his master called him before him and said to him, 'You wicked servant! I forgave you all that debt because you asked me; should you not have had mercy (eleo) on your fellow servant, as I also had mercy (eleo) on you?'
And his master was wroth, and delivered him into the hands of the torturers, until he should pay all that was owed him. This is also what my heavenly Father will do to you, unless you forgive each of your brothers from your heart."
Greek texts use the word "basanistes" which means "torturer". God cannot forgive the bitterness of the born again person, because bitterness means that this person does not believe in Jesus. The only way out of problems is to believe in full forgiveness of Jesus and forgive the other people completely.
If the soul of a believer gets stuck in bitterness, the spirit becomes imprisoned and the believer cannot enter the life, faith, freedom and blessings of God's kingdom. The believer's unforgiveness is darkness of the soul, but it does not take away his salvation, which is through the regenerated spirit. Otherwise, Jesus' work of atonement and great glad tidings would not be true.
You can check possible traces of bitterness studying these questions:Did you really receive and surrender to Jesus? Have you confessed and renounces all darkness in you life?
Has the Holy Spirit delivered and healed your soul, mind, feelings, memories, dreams and will power? Do you have bondages of witchcraft, false religions or other demonic power?
Have you received a proper teaching / revelation / understanding about forgiveness of Jesus? Have you reveived the Holy Spirit?
Do you have rebellion against Jesus? Do you have compelling addictions, lusts, oppression or loss of control of your life?
Do you have known of unexplained guilt in your heart? Do you have difficulties in dealing with accusations? Do you accuse other people?
Do you have thoughts of judgment, revenge, greed, selfishness and anger?
Deal with these questions in the light of the Holy Spirit. Don't listen to the accuser, because you are forgiven in Jesus. Confess and give your problems to Jesus. Then you have authority to judge, renounce, reject and dismiss the darkness. Believe you are forgiven of all these – once and for all. Jesus wants you to live in His complete freedom and forgiveness.