Thomas Pfau's comments can be applied to the general themes of the poem, but they are especially useful for our study of the poem's ending:
Conceiving of the subject from a genetic and personal, rather than transcendental and analytic perspective, Shelley links the emergence of self-consciousness to the awareness of a temporal discontinuity. The awareness of such a discontinuity does not, however, allow reflection to reconstruct the origin—the “one mind” of this discontinuous consciousness. On the contrary, self-consciousness itself is only conceivable as a formal device whereby we seek to contain the entropy, which characterizes all memory of ourselves, within the more controlled economy of cause and effect. That is, to speak of a self-consciousness, according to Shelley, implies a desire to fill “the chasm of an insufficient void, and seek to awaken in all things that are, a community with what we experience within ourselves” […]. Moreover, Shelley recognizes that a self-consciousness characterized by the very awareness of its own discontinuity cannot, in turn, be reified by an inward act of reflection. (102)
So Pfau is saying that one's awareness of one's discontinuity occurs within social circles. The desire to "intermix" "bosoms" (565) turns into a vivid fear--"Woe is me!" (587). Ultimately, language cannot deliver him from his own ego and self-consciousness, which makes the reality of even a sexual union that is beyond the symbolic to be quite threatening. The discontinuity, perhaps, is something Shelley ultimately wants to preserve due to the fact that his poetical survival has been dependent on his ability to maintain a strong, individualist political voice.
Pfau, Thomas. "Tropes of Desire: Figuring the 'Insufficient Void' of Self-Consciousness in Shelley's Epipsychidion." Keats-Shelley Journal 40 (1991): 99-126.