"Cease, cease" is a strange way to begin. The speaker seems already overwhelmed, pleading with someone, perhaps Constantia, perhaps his own racing mind, to stop before feeling swallows him. He calls the state he is slipping into a "wild lesson" that only madmen learn, and says that to be "lost" and "sink and die" in it "Perchance were death indeed." Then he turns, almost in mid-breath, to address Constantia directly. The rest of the fragment tries to say what her presence does to him, and keeps admitting that it can't be said.
The description relies on the senses, and Shelley passes the power of Constantia from one to the next. First comes sight: "In thy dark eyes a power like light doth lie." Then sound, though sound is exactly what has gone quiet, since her voice's notes are "laid to sleep." Next comes smell, as the power lingers "Within thy breath, and on thy hair / Like odour," and finally touch, when it "like fire doth leap" from her hand. Sight, sound, scent and touch each get a line or two, and by the end her influence seems to have no single source. Even silence doesn't switch it off.
Fire and water then collide in the tenth line: "my burning cheeks are wet." The speaker is feverish and weeping at once. "Even while I write" is the other striking phrase, because it makes the poem feel like live composition, with tears falling on the page as the lines form. The first stanza ends on a paradox: "the torn heart can bleed but not forget." Healing usually depends on forgetting, and the heart here is denied it. That line is also the longest in the stanza, twelve syllables against the ten of the opening, so the sentence seems to stretch out under its own weight.
The stanza's shape supports this. It opens in confident pentameter, with alternating rhymes on "learn" and "turn," "die" and "lie." The middle lines shrink to shorter, choppier measures, where the rhymes interlock (sleep/leap, hair/yet/wet) and the "-et" sound returns three times before the end. Some of that choppiness may come from the gaps in the printed text ("Between lips," "it is yet"), where words are missing and the editor's note admits the text has been pieced back together. We can't know what filled those blanks, but the effect is oddly fitting. A poem about feeling that exceeds language reaches us with holes in its language.
The second stanza, only four lines, makes the point more openly. It begins with "breathless awe like the swift change / Of dreams unseen but felt in youthful slumbers." The phrase "unseen but felt" turns the earlier sensory catalogue inside out. The experience is now less about perceiving Constantia than about a bodily sensation with no picture attached, like remembering the mood of a dream after the images have gone. The next line, "Wild sweet yet incommunicably strange," picks up "wild" from "wild lessons" in line one. Wildness first meant madness and now means sweetness, and Shelley seems to say the two can't be separated. The word "incommunicably" is a small irony, because the speaker uses a long, careful adverb to say that communication is impossible.
The last line, "Thou breathest now in fast ascending numbers," has a double meaning. "Numbers" was a common word for verses, so Constantia's breath rises as a quickening rhythm of poetry as well as of pulse. She seems to be both the inspiration and, in some sense, the poem itself. The stanza's alternating rhyme (change/strange, slumbers/numbers) is tidier than anything in the first, as though the verse is steadying itself, and then the ellipsis cuts it off. The fragment simply stops on a rising breath.
Part of what makes the piece worth reading is that it dramatizes a familiar Romantic problem: how can language hold an emotion bigger than itself? Shelley is one of the major Romantic poets, and this passage shows that preoccupation at small scale. The poem doesn't resolve the problem. It lets the strain show, in the paradox of the heart that bleeds without forgetting, the fire and tears, and the dream felt but unseen. The broken text almost proves its point.