According to Wikipedia, bitterness is "the experience of a negative emotion felt as a result of a real or imagined wrong done." Simply put, bitterness occurs when we won't let things go. Are we so naive as to imagine that life will always be fair and we will never be done wrong? Of course not. Life is life. We know how it works. Sometimes it's good, and other times it's not so good.
I love the old saying, "When life hands you lemons, make lemonade." (I've also heard the version: "When life hands you lemons, throw them back and ask for chocolate." Sounds good to me!)
Anyway, bad things are going to happen. Friends are going to let us down. Family members are going to forsake us. Bosses are going to promote "the other guy." These things are part of life. We all wish they weren't, but that doesn't change the facts. Life is full of ups and downs, goods and bads. It's up to us to deal with it!
I once heard a bitter person described as one who drinks a poison and waits for the other person to die. Bitterness doesn't hurt the person or situation that wronged us. It hurts us! It drains the joy out of life. It causes us to bear extra burdens that would be better off left far behind. Eventually, it will start to affect the people around us. Then we really have a mess!
During His time on Earth, Jesus spoke a lot about forgiveness. When we've truly forgiven someone, the bitterness will disappear. I'm not going to tell you that it's easy because it's not. Just like all our other spiritual housecleaning, removing the stain of bitterness can be quite a chore. But, it is possible, and the Lord will help us.
Follow peace with all men, and holiness, without which no man shall see the Lord: looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled. - Hebrews 12:14-15