Anger (or bitterness, hate, etc) is a very powerful emotion.

We're not very good at it either.

Wherefore, my beloved brethren, let every man be swift to hear, slow to speak, slow to wrath: For the wrath of man worketh not the righteousness of God.
- James 1:19-20 (KJV)

Anger is hard to let go of sometimes, and can take up valuable time.

It's important to understand that when there's anger in our heart, it stops us from loving fully.

It's better to ask God to transform the anger back into love again.

The most common way for that to happen, is to 'simply' forgive the other person.

Take the anger, and leave that with God.

God says that He will repay, and that vengeance is His.

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

Knowing that God will hold people accountable is more than enough for me.

It is a fearful thing to fall into the hands of the living God.
- Hebrews 10:31 (KJV)

It's also worth noting that making a distinction between a sinner, and a sin can be helpful. Love the sinner, but hate the sin, sort of thing.

But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you; That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.
- Matthew 5:44-45 (KJV)

Generally speaking, our enemies are the ones who don't know Jesus yet.

Were we so different once?

And you hath he quickened, who were dead in trespasses and sins; Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience: Among whom also we all had our conversation in times past in the lusts of our flesh, fulfilling the desires of the flesh and of the mind; and were by nature the children of wrath, even as others. But God, who is rich in mercy, for his great love wherewith he loved us, Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved;) And hath raised us up together, and made us sit together in heavenly places in Christ Jesus: That in the ages to come he might shew the exceeding riches of his grace in his kindness toward us through Christ Jesus. For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast. For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.
- Ephesians 2:1-10 (KJV)

It's perfectly alright to ask God to help you with all of this.

Forgiveness makes room for God's love.