We are sanctified by the Holy Spirit and are being sanctified continually. If you are having trouble forgiving this needs to be something you are actively praying about with patience. And in due season, you will receive an answer.
In the meantime, if one were to find themselves in a situation wherein they cannot forgive, then considering one's own sin in regards to the holiness of God is appropriate. Each of us have been forgiven a debt that we could not have paid, however our true opinion of how righteous we think we are without God's sacrifice often reveals itself in how little we are affected by God's sacrifice on our behalf.
Consider the scripture, that whosoever is forgiven much - loves much and whosoever is forgiven little loves little.
This is not implying that some people are more righteous than others but rather that some people recognize their soul's bankruptcy in the sight of God while some think themselves greater than they are.
Remember that our righteousness, not simply our mistakes, are as filthy rags.
"But we are all like an unclean thing,And all our righteousnesses are like filthy rags; We all fade as a leaf, And our iniquities, like the wind, Have taken us away."
If one cannot receive and understand this love God has for us in spite of our "ugly" condition, it will be made manifest in one's life through a lack of love for others and lack of ability to forgive.