Hebrews 12:14-15 NIV
It can be easy to let bitterness take over your thinking but you should try and resist it and trust God with your pain. If we examine forgiveness, we soon understand that the act of not forgiving someone is a deliberate decision. This usually fuels the desire to get even and is considered a problem. This effort is so filling of the soul that usually you focus most of your energy on retribution.
Remember, at your root is where the seed of resentment grows and the root feeds everything, so your resentment will soon spoil your life. If love and peace are your foundation, you will experience that growth from the root and your branches will bear good fruit.
Keep your heart pure and clear of harboring hate and you will find that when you filter your emotions through your heart, you will live Godly and without bitterness.