The Freedom of Forgiveness

Forgiveness is not a suggestion; it is a command therefore, refusing to do so is an act of disobedience.
“For if you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses”(Matthew 6:14-15).
If God has forgiven us, how can we refuse to forgive others?