A rose thrives in noon air and fountain water, then turns pale under the moon. From that one small scene Shelley builds a story of emotional damage. The first stanza reads like pure nature description, until the second begins "Such is my heart" and shows it was a comparison all along. The speaker's heart has been drained of colour by a cold influence, and the title points to who is responsible: a woman named Constantia.
The first stanza sets up its opposition carefully. Everything before the moon is soft and generous. The rose "drinks the fountain dew," a phrase that makes it sound contentedly alive, and the air is "pleasant," the hour noon. Then the verbs and colours turn. The rose "grows pale and blue" and becomes "wan," a word for sickly paleness. The moon is given a "gaze," which makes it a watchful, almost judging eye. The quiet centre of the stanza is the phrase "borrowed light." The moon shines with light that isn't its own, so it is "cold and bright," brilliant but without warmth. The poem suggests that beauty with no warmth behind it can be harmful. The rhyme of "dew" with "hue" in lines one and three puts the loss in a single pairing: moisture and nourishment give way to a changed colour.
The rhythm helps the effect. Lines carry roughly three or four beats, with shorter lines alternating with slightly longer ones, so the stanza sways like a song instead of marching. Its rhyme scheme is ababcc, and the closing couplet, "bright" and "light," lands like a verdict.
The second stanza begins with the same pattern, but it is more tangled. "Roses are fair, / And that at best a withered blossom" is hard to untangle, and the syntax seems to stumble as the speaker's feelings crowd in. "Blossom" and "bosom" are linked by their shared ending rather than a full rhyme, and the extra unstressed syllable closing those lines gives them a soft, falling finish.
Here the moon's borrowed light turns into "thy false care." Judging by the imagery, the woman's affection looked like warmth but, like moonlight, had none of its own. The most striking image is that she "did idly wear / Its withered leaves in a faithless bosom." A flower pinned to someone's breast is decoration, worn for a while and thought of lightly. The speaker's heart has been treated the same way. "Idly" makes the carelessness the real wound, since she did not even act out of cruelty. The "faithless bosom" is also a harsh place to end up, as the one spot that should be safest turns out to be unreliable.
There is a possible irony in the name. "Constantia" is the Latin word for constancy, and this poem accuses its addressee of being false and faithless. That is a reading offered from the title and the text, not a claim about the person behind the name, whom the poem never describes.
The last lines turn back to the first stanza: "And fed with love, like air and dew." The "pleasant air" and "fountain dew" now describe what love once did for the heart. The opening was a picture of the speaker's own earlier health, not just scenery. Then the text breaks off at "Its growth--". The sentence stops in mid-air, with the rhyme for "dew" still to come. Whatever the poem meant to say about that growth, we can only guess, though the damage done in the first stanza makes a hopeful ending hard to imagine.
The poem is worth reading for how little it says directly. The speaker never says "I am hurt" and never describes the woman's behaviour. A flower, a fountain and a moon carry the whole emotional argument, and the details of colour, temperature and borrowed light make the betrayal feel physical. The unfinished ending adds something too, because the poem stops where the sentence about recovery begins and leaves the reader with the withered rose.