Shelley is referring to people in his life who have become too tangible, too overwhelmingly influential on his thought because his sensitivity to their presence has inhibited his imaginative freedom to unify his self with other forms that are less tangible, such as Emily's. Specifically, Shelley is probably referring to his current wife--Mary. Teddi Chichester Bonca concentrates on Shelley's paradoxical need for unification and individuality: "For a feature of the Shelleyean sympathetically bonded universe is that, in a very real way, individual members of this vast network supply one another's atmospheres. While the influence one person sheds may be vivifying, another person may emit what Shelley calls in Epipsychidion 'a killing air'" (53).
In Epipsychidion Shelley seems to be exhibiting how the complete, aesthetic experience of unification is temporal, that there must eventually be a break away from such a unification--which he sees as necessary and regretful--in order for the individual "I" to persist. There may come a day, as Shelley hoped, when this restlessness ceases permanently, but the manifestations of his poetic impulses serve as proof that his placement within what Schelling calls the "world soul" through various conduits is not permanent. The "legislative" force of poetry, however, keeps such a hope alive for the reader and writer alike. The following passage from Bonca's introduction to her book, Shelley's Mirrors of Love, seems to support my claim:
No struggle was more central to his life or his work than that which pitted the demands of the Self against the claims of Love, and no vision was as passionately--if ambivalently--embraced as that of a world in which the streams of sympathy flowed unimpeded. His political opinions, his social theories, and his attitudes toward gender, sexuality, and sentimental love were all shaped and re-shaped by the ongoing dialectic that the powerful forces of Love and the Self created. (10)
Bonca, Teddi Chichester. Shelley's Mirrors of Love : Narcissism, Sacrifice, and Sorority. Albany: State U of New York P, 1999.