1 Peter 3:9,
Don’t repay evil for evil. Don’t retaliate with insults when people insult you. Instead, pay them back with a blessing. That is what God has called you to do, and he will bless you for it.
You have to find a way to show kindness to the people you have trouble forgiving. They don’t deserve it and probably won’t appreciate it. But you’re really not doing it for them, but for yourself. It will soften your heart.
So find something small: pray for them, smile at them, stop speaking negatively about them. Do it even though its painful. Do it out of devotion to Christ. Do it to receive a blessing in return.