Dickinson's poem "While I was fearing it, it came" uses the conventional ballad or common measure we expect to find in her poetry. This is the alternating lines of iambic tetrameter and iambic trimeter, with the rhyme scheme of abcb. This form suggests a familiar and comfortable pattern of sound and rhythm within which Dickinson works, usually to create some element of discomfort.
In this poem, the use of the pronoun "it" creates the discomfort. What precisely is the thing to be feared? We might imagine she means death, mental illness, or something else. Because it remains unknown, the shape of it cannot be constructed in the reader's imagination, leaving one to perhaps dread whatever one fears on one's own, to project one's own fear onto the poem. "It" then becomes a thing inside oneself, rather just an object within the poem.
Dickinson's ability to give profound psychological nuance to a human condition without seemingly working very hard to do so is one aspect of her brilliance. Whatever "it" might mean in this poem, it too seems to come upon the reader with the subtlety described within the work.