...Contrition is not genuine unless every mortal sin be detested.
It is impossible for some mortal sins to be forgiven and others to remain unforgiven.
ALL are pardoned or NONE is pardoned.
It is impossible for light and darkness to be in one and the same place.
Hence, Sanctifying Grace and mortal sin cannot dwell together.
If there be grace in the soul, there can be no mortal sin; and if there be mortal sin, there can be no grace for mortal sin expels all grace.
If Sanctifying Grace abides in the soul, the soul has a claim to Heaven.
If the soul is in the state of mortal sin, it is headed for Hell.
The sinner must therefore necessarily be sincerely sorry for all mortal sins if he wishes to be reconciled with God, for it is impossible to have a claim on both Heaven and Hell simultaneously; it is impossible to be a friend and an enemy of God at one and the same time.
In his epistle, St. James the Apostle states the principle thus: "Whosoever shall keep the whole law, but offend in one point, is guilty of all." (James 2:10).
One single moral sin retains the soul in the devil's power. And since no mortal sin is forgiven without sorrow, contrition must extend to all moral sins.
This, of course, does not mean one must make a special act of contrition for each individual mortal sin. It is sufficient that the act of contrition embrace all the mortal sins committed...
Excerpt from Confession: Its Fruitful Practice, Tan Books and Publishers