Let go of all unforgiveness before we pray, so that God can also forgive us. If we do not, God won't forgive us our sins.
If you are having trouble letting go and to forgive someone, take it to God in prayer, be honest with God, ask His help. Pray for those who hurt you, who did you wrong, pray with all your heart for them. It will change you, it will make you grow in your love for others, even your enemies, just like Jesus said, to love our enemies, pray for them, bless them. God will help you and fill your heart with His love for people.
God bless you.