The speaker of the poem is the only true character, so to speak, in this work. The speaker well may be the author himself in this case. Though I usually caution students not to assume that the speaker is the author, because this can lead us to make assumptions that cannot be supported by the text or even to miss meaning that we ought to see but is clouded by our assumptions about the speaker, because the narrator of this text is a poet, meditating on the process and practice of writing poetry, it seems relatively safe to conflate speaker and author here.
The speaker begins by describing the ways in which he feels like an "open house": with doors flung wide open for any and all who might wish to see inside him. He need not speak his "secrets" out loud because they reveal themselves in his work. He does this work because he "love[s]" it, but it renders him vulnerable, and the process is painful and difficult. The speaker feels that the process of making himself an "open house" and accessing the "foreknown" truths which he must discover and about which he writes require him to become "naked to the bone." Again, his sense of vulnerability and openness is front and center. It is as though he keeps himself purposely "spare" so that he can make room for "The anger [to] endure, / The deed [to] speak the truth."
He cannot even claim these for his own. The process is deeply spiritual, even mystical, as "language strict and pure" bubbles up through him to "stop the lying mouth": others who might seek to hide these truths rather than reveal them, as the poet does. It is painful, again, as the speaker talks about the "Rage" that "warps [his] clearest cry / To witless agony." It is clear that, for the speaker, the poet must engage in processes of discovery of the self—making it an "open house"—in order to ready himself for the "truths" that he is capable of revealing. These processes are, evidently, laborious, exhausting, and agonizing.