"And forgive us our debts, as we forgive our debtors" Matthew 6:12.
Why are we to forgive others? Because God forgave us.
Unforgiveness is like a yeast. It grows, and grows, and grows, until you don't recognize yourself anymore. The only remedy is forgiveness. This is the only way you can heal. Don't worry about their reaction to your forgiveness.
"But if ye forgive not men their trespasses, neither will your Father forgive your trespasses" Matthew 6:15.