The speaker receives, or imagines, a message from the past, and it doesn't deliver what they hoped for. The letter "respects no expectation of a better account," which suggests that whatever it says won't improve the story the speaker has been telling about that time. There is no comfort or rewriting of history here, only the same facts as before.
The speaker's response is calm but firm. They "clear" their mind of this kind of correspondence, as if tidying a room. Then comes the strange last line: "I will have to carve other dice." Dice stand for chance, so the speaker seems to be saying that the old game is finished, and a new one will need new tools. Carving suggests slow, handmade effort rather than luck.
The mood is quiet and a little wry. There's disappointment, but it is mixed with resolve. The speaker doesn't rage at the past; they simply stop waiting for it to change and turn toward making something new.