Five lines, three comparisons, and a speaker who seems to vanish as we read: Shelley's fragment is a small study in being undone by love. The first line states the condition outright, "I faint, I perish with my love!", and the rest hunts for pictures to match it. One caution first. In the text supplied, the second line has a gap after "whose", so a word or more is missing. I will leave that alone and work with what is actually on the page.
The central idea is that love here is not a wound but a dissolving. The speaker does not die violently. He thins out. Every image is of something with no hard edges to begin with: a cloud, mist, a wave. The verbs trace the same slope downward: "faint", "perish", "grow / Frail", "die", "fail". Even "grow" is bent out of shape. It hangs at the end of the first line as if something were about to expand, and then "Frail" arrives on the next line and turns growth into weakness. The phrase "with my love" is also slippery. It could mean because of love, in the company of love, or even that the speaker's love is perishing alongside him. The poem lets all three readings stand.
The construction is compact. The rhyme scheme is ababb: "grow" and "glow" pair up, then "pale", "gale" and "fail" share one sound. That triple rhyme works almost like a chain of cause and effect: a colour of fading, the force that scatters, and the result. Most lines run to ten syllables, but line four, "I die like mist upon the gale", is shorter, only eight. It reads like a breath running out, which suits mist, and then the final line stretches back to full length and closes more slowly. Line three begins with a stress on "Under", which gives the glow a slightly heavier, hanging start before the line eases into its rhythm.
The imagery moves through the elements. The cloud belongs to the sky, and it is lit by "the evening's ever-changing glow". Evening is a time of endings, and "ever-changing" means the light never settles, so whatever it falls on cannot hold a fixed shape or colour. Next come the wind and the mist, where "gale" gives the dispersing force a name. Finally the poem drops to the sea. The last image is the most interesting: a wave that fails "under the calm". We would expect a wave to be destroyed by a storm. Instead it is ended by stillness, because a wave is nothing but motion and calm erases it. Judging by the imagery, the poem suggests that the speaker's feeling is an energy that cannot survive being held still. The soft, liquid sounds (the l's of "glow", "pale", "fail", and the long vowel in "calm") add to the effect. There is also only one exclamation mark, in the opening line. After that the poem quietens, as the wave does.
"Fail" is the last word, and it carries more than one meaning. It describes a wave subsiding, but it also names a human failing of strength or voice. The title calls the piece a fragment, so the poem itself stops short, and its incompleteness oddly echoes what it describes. I cannot say whether that effect was intended, but it is hard to read the ending without feeling it.
What makes the poem worth reading is how much it does with so little. Shelley is counted among the Romantic poets, and the habit of using nature to express inner states is part of that tradition. Here nature is not scenery. The speaker merges with it until there is hardly a self left to separate from the weather. "Dying of love" is a worn phrase, but choosing images with no solidity makes it fresh again. The speaker is never described as a person with a body. He is only a pale shape fading in changing light, a bit of mist, a wave going flat. In five lines the poem asks how much of a person is left when feeling has taken everything, and it leaves the answer open.