The seed of bitterness can spring up evil intentions. The past owes us nothing, but we God everything for getting us through the issue. Bitterness can cause good people to be sinful.
Bitterness may have been used against you and you didn't respond. The Bible tells us that a kind word turns away wrath. Kind words used against us if we are bitter can be calming. Be better not bitter. Ask God to come into your heart and remove and break anything that cause bitterness.