That difficulty may seem more pervasive today, but people have been struggling with "a sin by any other name" since long before Charlotte Mason's time. This is how she describes it in Chapter 1, "The Court of Appeal": we prefer to cast ourselves as victims of unfairness rather than admit we're envious of others. If there's a way to avoid looking straight at our sin, we'll take it. As Mason says, little children already know how to play this game. C.S. Lewis allows several of his Narnia characters to moan about how hard-done-by they are, most notably Edmund and Eustace. But even Lucy and Peter have their moments of trying weakly to justify un-Aslan-worthy behaviour.
Mason says that the Conscience can be too easily convinced that we really aren't the ones who thought up sculpting a golden calf, or tasting forbidden fruit; or at least that we had a good excuse. It's even better if we can convince others to sin along with us, or to help us justify ourselves. Often at the end of a T.V. crime drama, the cornered villain will say petulantly, "I had to do it. That wicked person was going to tell what I did (or steal my husband, or get what I wanted, etc.), so don't you see that I really had no choice but to murder them?" Their attempts at self-justification sound hollow and twisted.
And we can't even blame our misled Conscience-judge, if we've been playing "lawyer tricks" in the courtroom. The real consequence of sin is that we haven't just wronged, we've wronged Someone.
"And then something made Peter say: 'That was partly my fault, Aslan. I was angry with him and I think that helped him to go wrong.' And Aslan said nothing either to excuse Peter or to blame him but merely stood looking at him with his great golden eyes." ~~ The Lion, the Witch, and the Wardrobe
Like Peter in Narnia (and his Scriptural namesake), it's only when we look The Truth in the face that we know how deeply we are loved.