Romans 12:19 Dearly beloved, avenge not yourselves but, rather, give place unto wrath; for it is written, Vengeance is mine; I will repay, saith the Lord.

I am struggling with this myself. I stay mad and forget to place my anger in God's hands. I will be the first to admit that this causes many unnecessary struggles in my life. God meant for us to enjoy life, not spend our time being bitter and miserable. Jesus wasn't crucified for us to live as angry, resentful, grouchy, unhappy people. He died so that we can have joy, peace, and prosperity. We only have to remember the previous verse and these next ones....

Ephesians 4:26-27 (26) Be ye angry, and sin not; let not the sun go down upon your wrath. (27) Neither give place to the devil.

The devil wants to take our joy. When we allow him to, we are submitting to him. We must do everything to prevent him from succeeding. We must give our anger to God, before the sun goes down on it.