The devil took a man into his armory, and showed him all the weaponry with which he besieged souls:
Lust, pride, the capital sins, etc.
Over in the corner of the room, a black chest half buried in chains caught the man's eye:
"What's in that chest," the man asked the devil.
The foul serpent formed a wry smile, and responded:
"This is my most deadly weapon, for use on those who have withstood all the rest."
"What is it?" asked the man.
"Discouragement," replied the devil. "It causes those to remove themselves from the battle which could not be forcibly conquered any other way."
Moral: Thank God for your profound contrition ("O happy fault"), and for the firm purpose of amendment which He has given you.