Fact 1: Cohabitating before marriage is a mortal sin.
Yes.
Fact 2: A single mortal sin, unrepented, will damn someone to hell for all of eternity.
Yes.
But when I talk to engaged couples, I try what works.
Threatening with hell just doesn't work for me.
Maybe it does with you.
Can you elaborate on what you mean by it "working"? By that, I mean, what constitutes the experience being a success: the couple proceeding with the formal, official Catholic marriage? Is that the determination of success? (Or conversely, a couple who, after going through the preparation process, determine that they are ill-suited for marriage, and so break up -- would that be considered a success?)