I decided to start reading some work of Lovecraft. Reading The Beast in the Cave, I got stuck at the first paragraph I encountered:
The horrible conclusion which had been gradually obtruding itself upon my confused and reluctant mind was now an awful certainty. I was lost, completely, hopelessly lost in the vast and labyrinthine recesses of the Mammoth Cave. Turn as I might, in no direction could my straining vision seize on any object capable of serving as a guidepost to set me on the outward path. That nevermore should I behold the blessed light of day, or scan the pleasant hills and dales of the beautiful world outside, my reason could no longer entertain the slightest unbelief. Hope had departed. Yet, indoctrinated as I was by a life of philosophical study, I derived no small measure of satisfaction from my unimpassioned demeanour; for although I had frequently read of the wild frenzies into which were thrown the victims of similar situations, I experienced none of these, but stood quiet as soon as I clearly realised the loss of my bearings.
I think I managed to understand each separate clause, but the logical relationships among them get me really confused.
I have no idea why the author used "yet", "for" and "but" at the places where I marked them in boldface. I also don't know why he wrote "my reason could no longer entertain the slightest unbelief", which means "my reason could not hold unbelief" if I'm not mistaken. Shouldn't he express something like "my reason could not hold belief"?
I'm feeling kind of frustrated now, could anyone help me figure it out?
Thanks to Greg Lee's answer, now I get a sense of what the author wants to convey. But I'm still not sure if I understand the exact function of these three words: "yet", "for" and "but". Is this "yet" related to "indoctrinated ... demeanour" (the sentence followed until semicolon) or "indoctrinated ... bearings" (the sentence followed until period)? Does this "for" stand for "because"? If so, I find this "because ... but ..." sentence structure really weird...