God's Word warns us against the dangers of bitterness.
Hebrews 12:15
James 3:14-16

Do not carry or keep bitterness, but replace it with love, kindness, and forgiveness.
Leviticus 19:18
Matthew 6:14-15
Matthew 18:23-35
Ephesians 4:31-32
1 Peter 4:8

Prayer and forgiving and forgetting are the cures for bitterness.
Mark 11:25
Romans 12:2
Philippians 3:13
Colossians 3:13