The poem uses evocative and sometimes ambiguous imagery to illustrate the speaker's confusion and pain. In the first line, for instance, the speaker says that the "hills step off into whiteness." This visual of a landscape shrouded in mist reflects the speaker's own sense of being lost and alone, overcome by an oppressive "fog" of despair. The speaker is subtly personifying the hills here as well, treating them as though they're taking a dangerous "step" into the unknown.
In the second stanza, the speaker describes a passing "train," saying that it "leaves a line of breath" as it cuts across the landscape. While the speaker never says what season the poem takes place in, this imagery might call to mind the way that one's breath looks in the cold, thereby casting a chill over the poem. The train, like the hills in the first stanza, also seems alive here, like a presence sighing past the speaker (perhaps in another gesture of "disappointment").
The speaker then switches gears, describing a "slow / Horse the colour of rust." This might refer to a literal horse (one that, presumably, the speaker is riding) or it might be a metaphor for the train. Either way, it seems to symbolize the speaker's own life, which feels to them like a creaky, plodding animal.
What's clear, above all, from this imagery is that the speaker feels as if they've fallen behind, that they've missed the "train" and are instead proceeding at an almost crawl, their horse practically "rust[ing]" beneath them as they go. They even describe the "[h]ooves" of this horse as "dolorous bells," sonic imagery that might make readers think of the mournful tolling of funeral bells. Perhaps the speaker fears moving so slowly that they'll die before they reach their goal—or perhaps they feel like they're already dead.
The speaker goes on to say that "the / [m]orning has been blackening" (or getting darker and gloomier), like "a flower left out." The world around the speaker seems as dark and dismal as their own mind. Like the horse, this flower symbolizes the speaker themselves: though once chosen—"picked" like a pretty flower—the speaker has since been forgotten, left to wilt and decay. Also note that both the morning and flowers usually suggest positive things like hope, renewal, youth, and vitality. The fact that these things are "blackening" and wilting thus implies that the speaker feels no hope. For the speaker, the future seems to have already withered on the vine.