Despair is certainty. Absolute certainty. Certainty that you have apprehended the universe in its entirety and found existence wanting.
Despair is a destructive self-delusion — the foolish conclusion that you have perceived the whole of everything. Hopelessness is the inability to see any possibility beyond the horizon, yes, but who are you to project your own blindness onto the cosmos? Who are you, ant, to profess such omniscience?
How did you reach this all-encompassing certitude? When did you decide that your knowledge reckoned and beheld the totality of all things? Congratulations on your laughable achievement.
When, my mind, did you assert your claim to infinity?
Depression, a disease — yes! The disease of certainty; a malignant growth of self-assured confidence. There is nothing for you, you say? Ah, tell me about this time you gazed upon the sum of omneity and justified your declaration!
What is hope, if not the conscious, humble acknowledgement that everything you perceive and contemplate is but a sliver of what is? How foolish, indeed, to project your fractional perspective onto everything you have yet to see and examine.
If despair be the disease, curiosity be the cure. Doubt, self-doubt, is the antidote. Be suspicious, mind, of your despair: for only an eye so foolish to claim that it has seen everything can claim that there is nothing worth being seen.