Bitterness

It is easy to get upset and angry about things in life. That's OK. But to keep dwelling on a situation and allowing anger to build day after day is a terrible thing. It leads to bitterness. Pauls says this in his letter to the Ephesian church:

"And “don’t sin by letting anger control you.” Don’t let the sun go down while you are still angry, for anger gives a foothold to the devil." Ephesians 4:26-27

There must be a time limit to your anger. By the end of the day you need to sort it out. Don't go to bed with it hanging over you.