Like depression and other emotional stress, bitterness and resentment can aggravate or cause physical problems such as arthritis. You can be affected mentally, spiritually, and otherwise. Your relationships will always suffer.
God can free you from this sin. It is an oppressive and destructive emotion having its root in hate, which is likened to murder. You must repent. No one can have peace and happiness with such emotions tearing at him. If you have not done so, ask God to forgive you and to come into your life right now. He will deliver you from the power of the enemy (Psalm 91:3).
If you are already a Christian, you should still ask God to forgive you for being bitter and resentful. Then ask Him to forgive anyone who may have hurt you, and toward whom you are bitter or resentful, even as He forgives you.
Seek to be baptized with the Holy Spirit if you are not already. Look for opportunities to demonstrate love to the person who offended you.
God forgives and forgets sin. However, you may have made your best effort to forgive and forget and find that you cannot. God can help you to cleanse your memory. Instead of remembering with malice and hurt, remember with forgiveness. Then go one step further and ask God to forgive your offender.
By forgiving and then asking God to forgive your offender, you release God to bless you and the other person.