Is Absolution Valid If You Don’t Know Something Is a Mortal Sin?

In order to be morally culpable for a mortal sin 3 conditions must be present (CCC 1857):

1. The action must be gravely wrong
2. The person must know it is gravely wrong
3. The person must freely choose the action

If someone doesn’t understand they have also committed other mortal sins then condition #2 is not present and the person is not gravely culpable (though they may still bear some moral culpability).