If end I gained It ends beyond Indefinite disclosed—
Looks like there's nothing in this stanza but more emptiness on the horizon—if there even is a horizon.
Here, the speaker tells us that every time she thinks she's found the end of something, she sees another end further beyond.
We could connect this back with the labyrinth reference in line 2: every time she turns a corner in the maze of nothingness, she just sees more nothingness ahead. (Hold up—are there corners in nothingness? Ugh, our heads hurt.) She might trick herself into thinking that it's about to end, but she's inevitably disappointed.
We also notice a bit of parallelism here. See how lines 6 and 7 are similar in structure, with the whole "If end" and "It ends" thing? Just like with the first line of the poem, we have an intentional repetition that gets across an idea of endless, empty monotony.
Line 8 reminds us of line 5 because it also contains two words that are just sort of plunked down in the poem with no regard to any kind of standard syntax.
"Indefinite" (8) also slant rhymes with "indifferent" (1.5), which doubly connects the lines in our minds.
Again, we have a line that seems disconnected in the same way that the speaker feels toward the world. But what does line 8 mean, exactly?
Well, "indefinite" describes something that's vague or unclear, and "disclosed" means to reveal a secret.
So, it could be that the line is describing the moments where she gets to what she thinks is an end in the nothingness, but then finds out that there's more nothingness to go.
Or the line could be describing the general realization she's having throughout the entire poem.
If so, it's a pretty terrible realization. It's like the speaker is saying, "Eureka! Nothing matters. Nothing matters at all."