Matthew 6:15
but if you do not forgive men their trespasses, neither will your Father forgive your trespasses. (RSVCE)
Your sins are only
absolved if they are
forgiven, and Jesus here says that one cannot receive forgiveness if they themselves will not forgive.
True repentance is a turning away from
all of our sins because they offend Him. If we repent of a few sins, but not a certain particular sin (in this case, withholding our own forgiveness from someone), then we do not understand the nature of repentance and have not done so.
Catechism 1431 says, “
Interior repentance is a radical reorientation of our whole life, a return, a conversion to God with all our heart, an end of sin, a turning away from evil, with repugnance toward the evil actions we have committed. At the same time it entails the desire and resolution to change one’s life, with hope in God’s mercy and trust in the help of his grace. This conversion of heart is accompanied by a salutary pain and sadness which the Fathers called animi cruciatus (affliction of spirit) and compunctio cordis (repentance of heart)”
It’s not enough to apologize to God for refusing someone forgiveness, either. You must have
repugnance towards that fault of yours,
and resolve to change your life. That means working at finally finding it in your heart to forgive.