Michael O'Neill, commenting on lines 588-91, writes:
Here the poetry's sense of its own inadequacy provokes a bravura show of self-construction; and yet this sense keeps in play the chance that "love's rare Universe" is other than the web of ropes which have sought to define the particular "Universe" of the poem. The disintegration but partial redemption of the poem's hopes is marked by the fourfold use of the first person in line 591. Perhaps the self cannot be one with the desired other. Yet, in the act of dramatizing this poem's expiration, Shelley sustains the possibility that, despite the failing of the self, the ideal still survives, albeit in a form unattainable through words. (142-43)
Not only does the ideal survive, but the poet may as well: "I pant, I sink, I tremble, I expire!" (591) may refer to emotionally-draining process of poetic creation, that writing is the subjective "pant" for the aesthetic object, followed by a sinking immersion, followed by a sense of annihilation, and then the annihilation, the mutability/end of the aesthetic/creative experience.
O'Neill, Michael. Percy Bysshe Shelley : A Literary Life. New York : St. Martin's P, 1990.