Because of the misunderstanding of justice, many Catholics cannot understand either mercy or love.
We all deserves just sentences for sin. None of us deserve mercy. That is the whole point about mercy. It is freely given. It is completely based on overlooking sin, not on denying sin.
A person who has received the mercy of God does so because of the salvific work of Christ on the Cross, not because of merit or good works.
Mercy is gratuitous. Justice is righteousness.
God is so pure, so good, so innocent, so holy that sin is a great injustice against Him. When He is merciful, it is simply because He loves us.
So, why cannot some people forgive?
We are forgiven.