Keats sought to understand much in his life; his poems and letters are full of urgent searching, of the kinds of questions that arise in the minds of passionate youth. He says in another letter:

You tell me never to despair—I wish it was as easy for me to observe the saying—truth is I have a horrid Morbidity of Temperament which has shown itself at intervals—it is I have no doubt the greatest Enemy and stumbling block I have to fear—I may even say that it is likely to be the cause of my disappointment.

For almost fifty years I have been troubled by depression.

It arises uninvited and unpredictably. It brings a morbidity of temperament that confuses truth, obscures beauty, and undoes goodness.  It is the cause of deep disappointment.

When I was younger I feared the onset of depression.  But I have learned to watch for its coming, abide its presence, and facilitate its passing.

Fear persists as powerless anticipation.  Watchful waiting is different.  Embracing the source of fear is neither fighting nor fleeing.

Fear signals that vulnerability and threat are about to intersect, as when the divorced parents of a beloved child meet at her wedding for the first time in many years, for the first time since he said to her, "I cannot live with you," and she replied, "You are disgusting."