What is Carol Ann Duffy trying to convey through her poem "Plainsong"?

In this poem, Duffy is conveying the indefinable sadness of returning to a place which "is your homeland" and yet in which you still feel out of place: "Lost One, Stranger who speaks with tears."
The poem describes a very particular moment in time: it is written in the second person, with the speaker commanding the reader to "stop" in the autumn woods, "straining" there to remember something that was once familiar and known. The character in the poem, "you," seems to have lost a lot of themselves, even "the words you have for things" no longer easily recalled. There is a sense that "you" should remember being here, in the "homeland," but the memories do not come. This sense of tension makes it "almost impossible to be here."
The grasses in the woods "are plainsong." Plainsong is a type of monophonic chant based on liturgies, usually associated with medieval times. To hear the grasses "chanting" plainsong in a forest suggests an association with this forest and earlier times; the liturgical element is echoed in the sound of "the evening bell reminding you, Home, Home." In the woods, then, this place feels like a country out of time—or, perhaps, "you" feel out of time within it, the bells and the plainsong making the autumnal wood feel as it might have done many years ago, and "you" unable now to fit in it.
"You" find it very difficult to be here in this wood, and yet there is a sense that the strangeness must be pushed through, as "you kneel, no one's child, absolved by the late sun." Again the word "absolved" has a liturgical connotation, suggesting a hope that this strange feeling can be made to dissipate—but in the hand is "a stone in your palm telling the time." This stone, anchoring, could be read as a suggestion that the woods can never be "home" again so long as the stone is there—as it "tell[s] the time," it is a continual reminder to "you" that, in this time to which they are tethered, they are a "stranger," lost in a place they feel they should understand, and yet which they cannot remember.