Ghislaine McDayter suggests that "how / Shall I descend, and perish not?" means that the poet wants a union with his idealized female "without losing his identity altogether" (32).
The issue of poetic identity was, of course, a major concern among romantic poets. In his letter to Richard Woodhouse, Keats proclaims himself to be a camelion Poet, who
is the most unpoetical of any thing in existence; because he has no Identity--he is continually in for--and filling some other Body--The Sun, the Moon, the Sea and Men and Women who are creatures of impulse are poetical and have about them an unchangeable attribute--the poet has none; no identity--he is certainly the most unpoetical of all God's Creatures. (501)
He distinguishes himself "from the wordsworthian or egotistical sublime" (500). We can, perhaps, think of Epipsychidion as wavering between characterless writing and the egotistical interest in self-identity.
McDayter, Ghislaine. "O'er Leaping the Bounds:
The Sexing of the Creative Soul in Shelley's Epipsychidion." Keats-Shelley
Journal: 52: (2003): 21-49.