The poem's metaphors help the speaker convey the pain of their predicament.
When the speaker explains that they "cannot live with" their beloved because they'd have to watch them die one day, their metaphors turn this imagined moment into something out of a horror story:
And I – Could I stand by
And see You – freeze –
Without my Right of Frost –
This image of freezing evokes the silence, stillness, and chill of a corpse. But what's really horrific here is the idea of seeing the loved one "freeze"—helplessly watching as rigid cold takes over the beloved body. The speaker imagines needing to claim their own "Right of Frost," killing themselves to follow their beloved; perhaps there's even a pun here suggesting a rite of frost, a ritual suicide.
Elsewhere, the speaker's adoration of their beloved takes the form, not of ice, but of water, "saturat[ing]" the speaker's "sight." Here, the joy of looking at the beloved seems to soak right through the speaker's eyes, an image that suggests both overwhelm—there's no room left for anything else in those eyes—and a kind of liquid aliveness, in contrast with the frozen stiffness of death.
Such an overwhelming love proves too frightening for the speaker, who decides at the end of the poem that they and their beloved can only "meet apart," maintaining a distant relationship. The sole "Door" between them, the speaker says, will come in the form of "Oceans," "Prayer," and "Despair." This final complex metaphor pushes the beloved away at the same time as it claims to hold a door open. Those oceans suggest almost insurmountable distances, while the "prayer" seems like a fairly chilly consolation, addressed to God rather than directly to the beloved.
"Despair," meanwhile, the speaker presents as a "White Sustenance," a pale (and perhaps rather anemic) food—not the red meat of committed love. If it's "sustenance," though, it provides some kind of nourishment; maybe despairing over an impossible love is one way for the speaker to cling to it, however painful that might be.