According to the dictionary, the meaning of a person being bitter is someone who carries resentment and is angry and disappointed at being unfairly treated.

There are scores of Christians who are bitter, just as the Bible says,

“Their mouths are full of cursing and bitterness.” Romans‬ ‭3:14‬ ‭NLT‬‬

“They will be unloving and unforgiving; they will slander others and have no self-control.”
‭‭2. Timothy‬ ‭3:3‬ ‭NLT‬‬

Jesus said that,

“God blesses you when people mock you and persecute you and lie about you and say all sorts of evil things against you because you are My followers. Be happy about it! Be very glad!” Matthew‬ ‭5:11-12‬ ‭NLT‬‬

The apostle Paul taught,

“Bless those who persecute you. Don’t curse them; pray that God will bless them.” Romans‬ ‭12:14‬ ‭NLT‬‬

That is why we are being told as followers of Jesus Christ to,

“Get rid of all bitterness, rage, anger, harsh words, and slander, as well as all types of evil behavior.” Ephesians‬ ‭4:31‬ ‭NLT‬‬

And also to,

“Look after each other so that none of you fails to receive the grace of God. Watch out that no poisonous root of bitterness grows up to trouble you, corrupting many.” Hebrews‬ ‭12:15‬ ‭NLT‬‬