Can't make sense of a paragraph from Lovecraft

Solution 1:

Yet makes an exception to the hopelessness: despite his grim fate, the narrator takes comfort in his lack of panic.

For introduces an explanation of why this would be comforting.

But contrasts what he did not do with what he did. A similar usage of but: “The Patagonian mara is not an ungulate but a rodent.”

These three conjunctions do not bear any special syntactic relation to each other; they're not like neither … nor, for example.

Solution 2:

"my reason could no longer entertain the slightest unbelief" means "I could no longer doubt". He was convinced that he would never again see the light of day. He goes on to say that he was pleased with himself that he could accept this terrible turn of events with equanimity.

The prose style is deliberately impenetrable.