"Catholics who are conversant with what the Church teaches will know when they are in a state of mortal sin".
But, remember that until 1965 (in the U.S. anyway) that included eating meat on a friday.
The U.S. bishops went ahead of most of the World in applying the dispensation, which created the ridiculous situation whereby what was a Mortal Sin in many countries ceased to be a sin at all the moment one boarded an American-flagged ship or aircraft at the local port or airport.
That was when the authority of the bishops started to unravel (Humanae Vitae gave it a good kick along of course).
The bishops kept on thinking that they could invent sins and enforce them just as they had done in Mediaeval times.
If you want to know why people hardly ever go to Confession/Reconciliation nowadays is not because they don't believe in it; it's because of the way it has been practiced by many priests, and also the fact that some priests took the cover of the Confessional to abuse children.