Matthew 6:14-15 – If you forgive those who sin against you, your heavenly Father will forgive you.  But if you refuse to forgive others, your Father will not forgive your sins. (NLT)
Forgiveness of others is a commandment, not an optional task.  It is a weapon that breaks every stronghold Satan throws at us.  If we do not forgive others for their sin against us, it eats away at us; our joy, peace, and tranquility.  We are the ones who lose sleep over it, not the trespasser.
Another way to look at it is that we have sinned against others.  There are people out there right now that may not have forgiven us, and we do not realize it.  Let’s hope they can find it in their hearts to forgive us, because we do not want them to suffer.  With the fact that we have sinned against others, we do not have the right to withhold forgiveness to those who hurt us.
Having an unforgiving heart has a ripple effect to those around us, and in our own lives.  For, if we have not forgiven others, God will not forgive us.  Another way to look at it is that we do not have God’s favor or blessing, we only have misery.  Lack of forgiveness will destroy relationships, create missed opportunities, and may even impact our family’s health and well being.
When we restrain our forgiveness, we are acting the part of God.  We cannot compare ourselves to such a perfect and wonderful God.  Our lack of forgiveness is blatantly arrogant, and it is sin in itself.  Besides, it gives Satan a hold onto our lives and tearing us down.
It can be understood if you choose to sever that relationship, because it may be toxic.  Forgiveness does not mean to be hit over and over again.  But it does mean you must hold onto resentment and bitterness; you should not seek revenge.
I think there is one thing we need mention which is often overlooked.  Not only should we forgive others for their trespasses, we should also forgive ourselves.  When you are forgiving, you are forgiven, and you are set free.