St. John Chrysostom Letters to St. Olympia.

Despondency is for souls a grievous torture chamber, unspeakably painful, more fierce and bitter than every ferocity and torment. It imitates the poisonous worm that attacks not only the body but also the soul, and not only the bones but also the mind. It is a continual executioner who not only tears in pieces one’s torso but also mutilates the strength of one’s soul. (Letter 10.2.b)

St. John Chrysostom 

Letters to St. Olympia