The speaker claims that, if he could have been a leaf or cloud on the West Wind, or felt young and powerful again, he wouldn’t be appealing to the West Wind now for its help.
He begs the wind to treat him the way it does natural objects like waves, leaves and clouds.
The speaker exclaims, "I fall upon the thorns of life! I bleed!"
He explains that the passage of time has weighed him down and bowed (but not yet broken) his spirit, which started out "tameless, and swift, and proud," just like the West Wind itself.
Finally, the speaker asks the West Wind for something: he wants the wind to turn him into its lyre.
This image is related to the æolian harp, a common metaphor in Romantic poetry. The æolian harp is sort of like a stringed version of a wind chime; it’s an instrument that you only have to put out in the breeze and nature will play its own tunes.
Here Shelley’s speaker describes himself as the harp, or "lyre," that the wind will play. He’ll be the instrument, and the West Wind will play its own music on him, just as it does in the branches of trees in the forest. That way, it won’t matter that he’s metaphorically losing his leaves.
The speaker and the trees of the forest are both decaying – the trees are losing their leaves, and he’s been bowed down by life.
But that doesn’t matter; if the wind plays both of them as instruments, they’ll make sweet, melancholy, autumn-ish music.
Now the speaker changes tactics; instead of asking the wind to play him like an instrument, he asks the wind to become him. He wants the wind’s "fierce" spirit to unite with him entirely, or maybe even replace his own spirit.
The speaker compares his thoughts to the dead leaves; perhaps the West Wind can drive his thoughts all over the world in the same way it moves the leaves, and they’ll become like a rich compost or mulch from which new growth can come in the spring. That way, even if his thoughts are garbage, at least that garbage can fertilize something better.
The speaker comes up with another metaphor to describe what he wants the wind to do to his thoughts, and this one isn’t about fertilizer. He describes his own words – perhaps the words of this very poem – as sparks and ashes that the wind will blow out into the world.
The speaker himself is the "unextinguished hearth" from which the sparks fly; he’s a fire that hasn’t gone out yet, but is definitely waning.
The speaker returns to the metaphor of the wind playing him as an instrument, but this time he describes his mouth as a trumpet through which the wind will blow its own prophecy.
The speaker ends by asking the wind a question that seems very simple: "If Winter comes, can Spring be far behind?"
The symbolic weight that he’s attached to the seasons, however, makes us realize that this is more than a question about the wheel of the year. He’s asking whether or not the death and decay that come at the end of something always mean that a rebirth is around the corner.
He’s hoping that’s true, because he can feel himself decaying.