Love, in this poem, is mostly guesswork. Dickinson's speaker cannot see the person she addresses, neither their face nor their "lifetime," so she has to imagine how often that person's heart aches for her. The poem never says why they are apart. "My far sake" tells us only that some distance lies between them. What the poem does is stay inside the uncertainty. She wonders, half-demands a "Confess," and then admits that the wondering hurts. Its central idea is that longing lives in the gap between imagining and knowing, and that imagination, the only bridge on offer, wounds the person who crosses it.
The first line sets this up with a pun. "You see I cannot see" begins as a conversational nudge, the "you see" of someone explaining herself, and then the same word turns literal. After that the poem fills with eyes: "the brave eyes film," and then "Mine -- got so dim!" The mirroring is quietly painful. She pictures the other person's eyes clouding with tears and finds that her own are already dim. A second pun sits at the hinge of the stanza: "But I guess guessing hurts." The offhand "I guess" collides with the poem's key verb, so a casual phrase becomes a flat admission that picturing the beloved brings no comfort.
The shape of the stanza acts out the fading. The opening line is long, but the next, "I must guess," is only three syllables. By the end the lines have dwindled to "Mine -- got so dim!", like a failing view. The rhyme is sparse and uneven. "Guess" and "Confess" match properly, while "film" and "dim" are only a slant rhyme, close but never quite clasped, which suits a speaker who cannot reach certainty. Dickinson's dashes break almost every line into gasps. Her grammar also slips under pressure: "How many times it ache for me" drops the verb ending, as if feeling has outrun correctness.
The second stanza turns from eyes to something more abstract and harder to pin down. Two lines open with the same word: "Too vague -- the face," "Too far -- the strength." The speaker's own qualities then take over. Her patience "covers" and her timidness "enfolds," verbs of wrapping, as though her restraint were a garment hiding both her face and the other person's strength. That is my reading, because the syntax lets us doubt what covers what. The effect is that her caution seems partly to blame for the distance.
The ending is stranger still. Something is "Haunting the Heart" (the capital H lends it weight) "Like her translated faces." Nobody called "her" has appeared before, and "translated" can mean changed, carried across, or lifted away. The imagery suggests that these faces are altered versions of the real one, which tease "the want" without satisfying it. Then comes the last line: "It -- only -- can suffice!" The word "only" is fenced in by dashes, so it stands alone. "It" is never named. I read it as the real presence, as opposed to any imagined face, but the poem withholds that answer. The poem's two exclamation marks fall at its two endings, one on loss ("so dim!") and one on need ("suffice!").
This is what makes the poem worth reading. Dickinson, who left close to 1,800 poems, most of them unpublished in her lifetime, was a master of compression, and here compression becomes the subject. Because "her" and "it" are never explained, we are placed where the speaker stands, guessing at what we cannot quite see. The poem does not describe the pain of imagining someone far away so much as hand it to the reader. The form, with its fraying grammar, shrinking lines and near-rhymes, is part of the feeling.