"Droll thing life is -- that mysterious arrangement of merciless logic for a futile purpose. The most you can hope from it is some knowledge of yourself -- that comes too late -- a crop of unextinguishable regrets. I have wrestled with death. It is the most unexciting contest you can imagine. It takes place in an impalpable grayness, with nothing underfoot, with nothing around, without spectators, without clamour, without glory, without the great desire of victory, without the great fear of defeat, in a sickly atmosphere of tepid skepticism, without much belief in your own right, and still less in that of your adversary. If such is the form of ultimate wisdom, then life is a greater riddle than some of us think it to be. I was within a hair's-breadth of the last opportunity for pronouncement, and I found with humiliation that probably I would have nothing to say. This is the reason why I affirm that Kurtz was a remarkable man. He had something to say. He said it. Since I had peeped over the edge myself, I understand better the meaning of his stare, that could not see the flame of the candle, but was wide enough to embrace the whole universe, piercing enough to penetrate all the hearts that beat in the darkness. He had summed up -- he had judged. 'The horror!'"
Although it is not the standard interpretation, I think that "wrestling with death" here refers to a suicidal crisis, and not a bout of fever. A few lines later Marlow notes that he "had been permitted to draw back my hesitating foot," unlike Kurtz, who had "made that last stride [and] stepped over the edge." That suggests far more intentionality than merely succumbing to a fever. I'm not sure, but Conrad might have more characters die by suicide than any other author, and he wrestled with suicide himself.
It's quite the understatement to note that Conrad bears re-reading. When I read him I often find myself re-reading pages I have just finished, for the sheer pleasure of the prose, and to let the import of his words sink in. If you have read it before, remember that you are not the same person now as you were when you first read it, so reading it again will be a new experience for you.
I would certainly consider Heart of Darkness and The Secret Sharer essential reading for aspiring clinical psychologists. I would add The Secret Agent, which is one of my favorite novels. Everyone one knows that Coppola and screenwriter John Milius were inspired by Conrad when crafting Apocalypse Now. Coppola's wife has her own remarkable documentary on the making of that movie, Hearts of Darkness, which I strongly recommend.