Once again Do I behold these steep and lofty cliffs, That on a wild secluded scene impress Thoughts of more deep seclusion; (4-7)
The speaker is back on the banks of the Wye, and the view is staggeringly beautiful. The "steep and lofty cliffs," especially, inspire him with awe. Perhaps it's because of their height and dramatic steepness that he is particularly drawn to the cliffs.
Nor less, I trust, To them I may have owed another gift, Of aspect more sublime; that blessed mood, In which the burthen of the mystery, In which the heavy and the weary weight Of all this unintelligible world, Is lightened: (35-40)
The speaker's memory of the "beauteous forms" (22) not only cheers him up when he's feeling bummed, but also makes him feel as though the weight of the world "is lightened." He calls the sensation a "blessed mood." This is the first hint of the almost religious devotion that the "presence" in Nature will inspire in him.
that serene and blessed mood, In which the affections gently lead us on,— Until, the breath of this corporeal frame And even the motion of our human blood Almost suspended, (41-45)
When the speaker remembers the "beauteous forms" (22), he's not so distracted by the immediate sensations of the "steep and lofty cliffs." He's able to reflect more. And that thoughtful reflection leads to the almost meditative trance that he describes here. His breath slows down and even his heart rate seems to drop.
we are laid asleep In body, and become a living soul (45-46)
The speaker's trance-like state allows him to become more spiritually awakened. His body is irrelevant, and he "become[s] a living soul."
While with an eye made quiet by the power Of harmony, and the deep power of joy, We see into the life of things. (47-49)
The harmony, or beautiful balance, that the speaker remembers in the "beauteous forms" makes his "eye" "quiet." He's not distracted by his physical surroundings. There could also be a pun in these lines. His physical "eye" is "made quiet," but so is his "I". His sense of himself, or his ego, is quieted. He can see beyond himself when he's in this meditative state.
And I have felt A presence that disturbs me with the joy Of elevated thoughts; a sense sublime Of something far more deeply interfused, (93-96)
The speaker has learned to sense a "presence" in nature. It's awe-inspiring, but it's also "disturb[ing]." Why "disturb[ing]" if it fills him with "joy"? Maybe because it's so "sublime" – so great and so incomprehensible – that he has trouble processing it. That could be "disturb[ing]."