Here, again, are the last two stanzas:
When hearts have once mingled,The important thing to remember abut Shelley is the supernatural delicacy of his effects ("Ah, sister, desolation is a delicate thing"). You can see that delicacy in the mingled interchange of reference. As the opening stanzas make clear, this poem is an example of a particularly Shelleyan version of allegory. "The light in the dust lies dead" can't quite mean that you can see light in the dust of the ground (as though the oil were still burning), because the light is dead. It almost means that, but it means that only if you understand that "the light in the dust" also and more deeply means: the light of life and love that animates the dust that we are, to which we will return. We've lost our spark, alive but loveless. The light in the dust that is still biologically alive has been extinguished.
Love first leaves the well-built nest;
The weak one is singled
To endure what it once possessed.
O Love! who bewailest
The frailty of all things here,
Why choose you the frailest
For your cradle, your home, and your bier?
Its passions will rock thee,
As the storms rock the ravens on high;
Bright reason will mock thee,
Like the sun from a wintry sky.
From thy nest every rafter
Will rot, and thine eagle home
Leave thee naked to laughter,
When leaves fall and cold winds come.
Allegory figures through externalization, so the allegorical figuration of that is light actually lying dead in the dust, as on a battlefield, exteriorized. This is not quite an image, though it sounds like one. It's a verbal substitution for an image, a verbal form which can never be imagined as a pure image, since you would have to imagine light and its extinction at once. I do imagine that, but only by personifying the image of light prone and dead somehow in the dust, light still but dead, like Burke's not quite imaginable "angel of the Lord."
This is the mode of the last two stanzas as well. After love has expressed itself (when the hearts have mingled and mingle no longer, "when the lips have spoken), it does not last. Not for both: the weak one still loves though love is over. This is the meaning of the self-discrepancy of the light lying dead in the dust. Love is dead, the weak one still loves.
The well-built nest decays when love leaves it. Who leaves? The one who doesn't love anymore, and so: love departs. Who stays? The one who loves, debilitated by loss, and so: love stays. It does both at once. Each is the equivalent of the other. Love's departure brings out the love felt by the one who's lost all, and from whom love can never depart.
Love doesn't return to its eagle home; its only nest now is the place where the weak remains forlorn since love is gone. Love is gone, and what's left is love. Allegorical exteriorization is just the right figure for this discrepancy within experience: what is inside can only be seen now from the outside, and that fact tells powerfully and devastatingly on inner experience and brings us back to inner experience.
So in the last stanza, the pronouns are only hard to fix if you think they have to be fixed. "Thee" is Love, addressed already in the second person in the previous stanza: "Why choose you the frailest / For your cradle, your home, and your bier?" So "its passions" are the passions of the "weak one" who must "endure what it once possessed."
And yet: it's the passions of love that must be rocking the weak one, who endures it. The change in antecedent this reading implies is confirmed by the shift from "you" ("Why choose you the frailest") to "thee," the weak one.
The final, deepest allegorical figure of love is the person whom love abandons, leaving that person to be love's image, the frail and sorrowful personification of love's frailty. What's left when love is faithless and abandons you is the truest image of the faithfulness of love, faithful love.
"What may I do when my master feareth, But in the field with him to live and die? For good is the life ending faithfully."