John Slater writes:
It is the poetic expression of this movement that gives “Epipsychidion” its special quality. In one sense, the poem records Shelley’s practical estrangement from the public at large; but it also demonstrates that Shelley experienced the ultimate aesthetic frustration of every writer, a sense of alienation between precious but inarticulate impressions and the rude currency of language. Between the extremes of capitulation to popular clamor and a vow of silence lies a third possibility at least as agonizing, a recognition that a poet can no more expect to find connubial bliss with an audience outside his own consciousness that the tenor of a metaphor can expect to become one with its vehicle. "Epipsychidion" testifies, however, that the narrative of such foredoomed struggles can culminate in literary triumph (292).
In the last stanza of the poem, Shelley admits publicly his failure to properly cultivate a private vision between himself and Emilia. His expressions of frustration, particularly at the end, read as tragic soliloquies intended for an audience rather than for one person. As Slater points out, while the poem is about what I would call the Wordsworthian quest of passivity and seclusion, the proactive work--the employment of syntactical strategies while implementing an imaginary plan of escape--fails. His "defeat," Slater argues, is a literary achievement partially because it mimics the futile artistic efforts of others. As Shelley intended this poem to be read mostly by fellow artists, it would be interesting to read an article that discusses the intent for the poem. Or, perhaps the intent evolved during the process of the composition. Would a successful, non-metafictional poem have won him a larger audience?
Note: Evidently, the quotations Shelley employs--"'We are the masters of thy slave; / What wouldest thou with us and ours and thine?'" (593-94) and 'Love's very pain is sweet, / But its reward is in the world divine / Which, if not here, it builds beyond the grave'" (596-98) are of Shelley's own. In fact, one can do a "Google" search and discover that this quote has been circulated a number of times. The language has a prophetic, Shakespearean quality to them, and I was surprised to discover that Shakespeare was not the source of them. Why is he quoting himself?
Slater, John F. "Self-Concealment and Self-Revelation in Shelley's 'Epipsychidion.'" Papers on Language & Literature 11 (1975): 279-92.