Get rid of all bitterness, rage, anger, brawling, and slander, along with every form of malice. That statement comes from Ephesians 4, verse 31. The Greek word for bitterness “Pikria” translates to “Sticky.” The brothers and sisters of bitterness are anger and disappointment. Often bitter people feel they are being treated unfairly whether or not that is the actual case. Bitterness causes resentment. That leads to a family of ill-feeling, spitefulness, malevolence, maliciousness, animosity, hatred, vindictiveness, vengefulness, hostility. It makes you want not to seek revenge but take revenge.
Bitterness and not forgiving is like holding a lump of red-hot coal in your hand with the intention of throwing at the target of your revenge. But all the time you are holding it, who is the hot coal hurting. You! Whether you know it or not.