Then my heart it grew ashen and sober As the leaves that were crispèd and sere— As the leaves that were withering and sere,
All of a sudden, the speaker gets it. In a second, his heart gets as cold and withered and lifeless as the leaves he mentioned at the beginning of the poem.
That's the third time Poe has repeated some version of these lines, for those who are counting. It becomes like a chorus in a song, helping the poem to come together as a whole.
And I cried—"It was surely October On this very night of last year
As soon as he hears the name of Ulalume, the speaker's memory comes flooding back. He realizes that it was exactly a year ago, on this very day in October, when he last came to this very spot.
If things seemed foggy and dreamlike before, suddenly they are terribly real and clear.
That I journeyed—I journeyed down here— That I brought a dread burden down here—
Now, as it all comes back, he remembers making a trip to this same place. When he made that "journey," he carried a "dread" (terrible, horrifying) "burden" (weight) with him.
He's being a little subtle and mysterious here, but putting everything together, we realize he must have carried the body of his dead love, Ulalume, down to this very tomb.
On this night of all nights in the year, Oh, what demon has tempted me here?
The speaker starts to wonder if this was just a coincidence. How could it be that he wound up in this same spot, exactly a year later, "on this night of all nights"? He wonders if he was lead down here by some "demon."
This is a big swing. Remember how, just a few lines ago, he thought he was being lead down the path by a kind, loving goddess? Now things have completely flipped and he's getting kind of paranoid.
Well I know, now, this dim lake of Auber— This misty mid region of Weir— Well I know, now, this dank tarn of Auber— In the ghoul-haunted woodland of Weir."
By this point, his memory is completely back. Now, as he looks around at the lake and the trees, everything is completely familiar. He's been here before, and he knows it.
The last time he mentioned Auber and Weir (lines 26-29), he was telling us all how his "treacherous memory" had deceived him. Now we know what that was about, and the mystery is solved.
When these lines repeat, they seem familiar, but also new. It's like an echo moving through the poem, changing a little as it goes.