Definition: Holding on to or showing feelings of intense [strong] animosity [hatred, anger], resentment or vindictiveness [wanting to get back at someone]. — Other words that describe bitterness are: merciless, unforgiving, holding a grudge. Bitterness is  also described as feelings resulting from something that is difficult to accept.

1. God’s Word warns us against the dangers of bitterness.

Proverbs 18:19 [When we allow ourselves to become bitter against others, we create walls difficult to overcome, which isolates us from others] A brother offended is harder to be won than a strong city: and their contentions are like the bars of a castle.
Acts 8:22,23 [Bitterness is a sin to be repented of] Repent therefore of this thy wickedness, and pray God if perhaps the thought of thine heart may be forgiven thee. 23 For I perceive that thou art in the gall [poison] of bitterness, and in the bond of iniquity.
Colossians 3:19 Husbands, love your wives, and be not bitter against them.
Hebrews 12:15 [Be on guard against it, because even a little root of bitterness can do much harm to you and others] 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 [contaminated]. [See also Deuteronomy 29:18]
James 3:14 If ye have bitter envying and strife in your hearts, glory not [don’t brag about it], and lie not against the truth.

2. Do not carry or keep bitterness, but replace it with love, kindness and forgiveness.

Leviticus 19:18 Thou shalt not avenge, nor bear [carry or keep] any grudge [bitterness] against the children of thy people, but thou shalt love thy neighbour as thyself: I am the Lord.
Proverbs 17:9 [Love can overcome bitterness, but watch out about opening up old hurts felt from others. Better to forgive and forget] He that covereth a transgression seeketh love; but he that repeateth [harps about] a matter separateth very [true] friends. [See also 10:12]
Matthew 6:14,15 For if ye forgive men their trespasses, your Heavenly Father will  also forgive you: 15 But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.
Matthew 18:23-35 [In His parable of the unmerciful servant, Jesus made it clear that we will suffer if we refuse to sincerely forgive our brethren, when we know Jesus has had mercy on our sins] 35 So likewise shall My Heavenly Father do  also unto you, if ye from your hearts forgive not every one his brother their trespasses.
Romans 2:1-6 [Hypocritical, judgmental self-righteousness can cause us to harbor bitterness, while feeling justified for doing so, yet it ends up bringing God’s punishments upon ourselves] Therefore thou art inexcusable, O man, whosoever thou art that judgest: for wherein thou judgest another, thou condemnest thyself; for thou that judgest doest the same things. 2 But we are sure that the judgment of God is according to truth against them which commit such things. 3 And thinkest thou this, O man, that judgest them which do such things, and doest the same, that thou shalt escape the judgment of God? 4 Or despisest thou the riches of His goodness and forbearance and longsuffering; not knowing that the goodness of God leadeth thee to repentance? 5 But after thy hardness and impenitent heart treasurest up unto thyself wrath [you are saving up terrible punishment for yourselves] against the Day of Wrath and Revelation of the righteous judgment of God; 6 Who will render to every man according to his deeds.
Ephesians 4:31,32 Let all bitterness, and wrath, and anger, and clamour [shouting or crying], and evil speaking, be put away from you, with all malice [desire to do harm]: 32 And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ’s sake hath forgiven you.
1 Peter 4:8 [Even if someone has mistreated or wronged you, God’s love is love enough to forgive] And above all things have fervent love among yourselves: for love shall cover the multitude of sins.

3. If you allow bitterness a place in your heart, it will eventually come out of your mouth in murmuring and complaining.

Job 6:2,3 [Job complains to his friends:] Oh that my grief were throughly weighed, and my calamity laid in the balances together! 3 For now it would be heavier than the sand of the sea: therefore my words are swallowed up [have been rash]. [See also 7:11]
Job 10:1 [Job also said:] My soul is weary of my life; I will leave my complaint upon myself; I will speak in the bitterness of my soul. [See also 16:6-16]
Job 23:1-4 [Job’s self-righteousness caused him to become bitter against God, thinking he knew better] Then Job answered and said, 2 Even to day is my complaint bitter: my stroke is heavier than my groaning. 3 Oh that I knew where I might find Him! That I might come even to His seat! 4 I would order my cause before Him, and fill my mouth with arguments.
Matthew 12:34b Out of the abundance of the heart the mouth speaketh. [See also 15:11,18,19]
Romans 3:14b Whose mouth is full of cursing and bitterness.

4. The Lord, in His love, can deliver you from the sin of bitterness. Here are some examples of three who overcame it, and one who didn’t:

Genesis 45:3-15 [If anyone had cause to be bitter, it was Joseph. His brothers had despised him; they brutally sold him into slavery and lied to their father that he had been killed by a wild animal; he spent time in prison on false charges of rape; was left there for two years longer because Pharaoh’s forgetful chief butler forgot his promise to help get Joseph out of jail. In spite of all this, here’s how Joseph reconciled with his brothers:] And Joseph said unto his brethren, I am Joseph; doth my father yet live? And his brethren could not answer him; for they were troubled at his presence. 4 And Joseph said unto his brethren, Come near to me, I pray you. And they came near. And he said, I am Joseph your brother, whom ye sold into Egypt. 5 Now therefore be not grieved, nor angry with yourselves, that ye sold me hither: for God did send me before you to preserve life. 6 For these two years hath the famine been in the land: and yet there are five years, in the which there shall neither be earing nor harvest  7 And God sent me before you to preserve you a posterity in the earth, and to save your lives by a great deliverance. 8 So now it was not you that sent me hither, but God: and He hath made me a father to Pharaoh, and lord of all his house, and a ruler throughout all the land of Egypt. … 10 And thou shalt dwell in the land of Goshen, and thou shalt be near unto me, thou, and thy children, and thy children’s children, and thy flocks, and thy herds, and all that thou hast: 11 And there will I nourish thee; for yet there are five years of famine; lest thou, and thy household, and all that thou hast, come to poverty. … 15 Moreover he kissed all his brethren, and wept upon them: and after that his brethren talked with him.
1 Samuel 1:10 [Hannah, mother of Samuel, was sorely provoked every year by Peninnah, the second wife of her husband, Elkanah.  Peninnah had children and purposely tried to make Hannah feel badly because she had no children. After years of Peninnah doing this, Hannah, having become bitter, not eating well and feeling that she somehow was not blessed of the Lord, took her bitterness to the Lord in desperate prayer, openly confessing her grief, both to the Lord, and to Eli, the priest, and finally a great victory was one, first by faith, followed by the Lord’s blessings] 10 And she was in bitterness of soul, and prayed unto the Lord, and wept sore. … [Eli, when he saw Hannah sobbing, mistakenly though she’d been drinking and was drunk:] 15 I am a woman of a sorrowful spirit: I have drunk neither wine nor strong drink, but have poured out my soul before the Lord. … [She explained her prayer that God give her a son] 17 Then Eli answered and said, Go in peace: and the God of Israel grant thee thy petition that thou hast asked of Him. 18 And she said, Let thine handmaid find grace in thy sight. So the woman went her way, and did eat, and her countenance was no more sad. [By faith Hannah had given her bitterness to the Lord, and because of it, was able to return to her family, having been freed from sadness and was even able to eat normally. Later the blessings came: the Lord not only blessed her with Samuel, her first born who she gave to the Lord, but  also with three other sons and two daughters!]
Isaiah 38:10-17 [King Hezekiah grew bitter during a nearly fatal illness. During his worst days, he felt the Lord was cutting him off. Yet his desperate prayer and tears brought eventual healing and victory] I said in the cutting off of my days, I shall go to the gates of the grave: I am deprived of the residue of my years. 11 I said, I shall not see the Lord, even the Lord, in the land of the living: I shall behold man no more with the inhabitants of the world. 12 Mine age is departed, and is removed from me as a shepherd’s tent: I have cut off like a weaver my life: He will cut me off with pining sickness: from day even to night wilt Thou make an end of me. 13 I reckoned till morning, that, as a lion, so will He break all my bones: from day even to night wilt thou make an end of me. 14 Like a crane or a swallow, so did I chatter: I did mourn as a dove: mine eyes fail with looking upward: O Lord, I am oppressed; undertake for me. 15 What shall I say? He hath both spoken unto me, and Himself hath done it: I shall go softly all my years in the bitterness of my soul. 16 O Lord, by these things men live, and in all these things is the life of my spirit: [then Hezekiah begins to speak faith:] so wilt Thou recover me, and make me to live. 17 Behold, for peace I had great bitterness: but Thou hast in love to my soul delivered it from the pit of corruption: for Thou hast cast all my sins behind Thy back.
2 Samuel 13:22 [Absalom, son of King David, bitterly hated his half-brother, Amnon, for having raped Tamar, Absalom’s full sister. This unbridled bitterness grew for two years until Absalom had Amnon killed] And Absalom spake unto his brother Amnon neither good nor bad: for Absalom hated Amnon, because he had forced his sister Tamar. … 28 Now Absalom had commanded his servants, saying, mark ye now when Amnon’s heart is merry with wine, and when I say unto you, Smite Amnon; then kill him, fear not: have not I commanded you? Be courageous, and be valiant. 29 And the servants of Absalom did unto Amnon as Absalom had commanded. Then all the king’s sons arose, and every man gat him up upon his mule, and fled.

 5. Bitterness stems from feelings of anger and an unforgiving attitude towards others. Here are some more solutions from the Word on how to avoid it.

Proverbs 27:7 [If we are truly hungry to learn from the Lord, even bitter experiences are looked upon as “blessings in disguise.”] The full soul loatheth an honeycomb; but to the hungry soul every bitter thing is sweet.
Matthew 5:23,24 Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought [grievance] against thee; 24 Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift.
Mark 11:25 And when ye stand praying, forgive, if ye have ought against any: that your Father  also which is in Heaven may forgive you your trespasses.
Romans 8:28 [Bitterness is  also described as the feelings resulting from something that is difficult to accept; so it’s important to always remember that the Lord has a good purpose in everything that He allows to happen to us] We know that all things work together for good to them that love God, to them who are the called according to His purpose.
Romans 12:2 [God’s Word often speaks of “renewing our minds,” which implies letting go of the old; particularly any past grievances and bitternesses] And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God. [See also Ephesians 4:23]
Ephesians 4:26b,27 [Going to bed in anger is an avenue for the Devil to work] Let not the sun go down upon your wrath; 27 Neither give place to the Devil.
Philippians 3:13 [Bitterness is  also caused by holding on to bad or angry feelings about someone or something that happened, but the Lord tells us to forget the things that are past] Brethren, I count not myself to have apprehended: but this one thing I do, forgetting those things which are behind, and reaching forth unto those things which are before.
Colossians 3:13 Forbearing one another, and forgiving one another, if any man have a quarrel against any: even as Christ forgave you, so  also do ye.