We are all of us in error, the humorists excepted. They alone have discerned, as though in jest, the inanity of all that is serious and even of all that is frivolous.
To try curing someone of a "vice," of what is the deepest thing he has, is to attack his very being, and this is indeed how he himself understands it, since he will never forgive you for wanting him to destroy himself in your way and not his.
Get hold of yourself, be confident once more, don't forget that it is not given to just anyone to have idolized discouragement without succumbing to it.
How can you know if you are in the truth? The criterion is simple enough: if others make a vacuum around you, there is not a doubt in the world that you are closer to the essential than they are.
As soon as one returns to Doubt (if it could be said that one has ever left it), undertaking anything at all seems not so much useless as extravagant. Doubt works deep within you like a disease, or even more effectively, like a faith.