In the Catholic Dictionary written by Fr. Hardon, it says this about mortal sin:
"It is not up to a human being to decide subjectively whether a deliberate serious sin is also a mortal sin that deprives him or her of God’s friendship. God alone has the right to determine what separates a sinner from the Creator; a creature does not have the right to stand in judgment on god and tell him what constitutes mortal sin."
What exactly is meant by a person cant judge that a serious sin they committed with full deliberation is mortal? Is that literally what mortal sin is? Fr. Hardon then says this, which seems to contradict the first statement:
"Every serious sin, therefore, is a mortal sin when a person freely decides to do whatever he knows God forbids under penalty of exclusion from the kingdom of heaven."