Agnes Peter argues that the poet "internalizes the Platonic quest for union with the divine He creates an image, which ostensibly is a mortal woman, but actually functions as a metaphor for his own immortal aspirations, 'a soul out of [his] soul,' [...] and antiself'" (123).
But by "internalizing the Platonic quest" Shelley is attempting to reverse Platonism, declaring his inner creative nature to possess divine powers that collapse the hierarchical binary between the self and the divine "other." By projecting his inner "antiself" outwardly, the poet is actively asserting a poetic will that wishes to mold/create an atmosphere to his own liking. This pursuit becomes agonizing for him as he realizes that such a poetic activity cannot be completely satisfying. As Agnes puts it, "[t]he poet wants to embrace the creature of his own creative imagination, but the attempt is a frustrating experience. Shelley's frustration can be explained on three different planes" (122).
Agnes Peter. "A Hermeneutical Reading of Epipsychidion." Keats-Shelley Journal 42 (1993): 120-27.