Besides the interpretations already explained in other posts, it is also possible that the narrator is telling the truth about the events of the story. Many of Poe’s stories contain elements of the supernatural, so I don’t think we can discount the possibility that the House of Usher—both the family and the physical dwelling—is cursed.
Before even meeting Usher, the narrator perceives the vapor that seems to emit from the ground and surround the structure. This could suggest that Usher’s “superstitions” about the house are not just a figment of his imagination but rather observable phenomena.
With this interpretation in mind, the narrator could be experiencing “symptoms” that are more related to the house itself than to whatever mysterious illness from which Usher suffers. The narrator, then, cannot escape the nefarious influence of the oppressive house, which might literally be causing such an uneasy feeling.