The speaker describes self-improvement as something closer to damage than progress. To "hew myself raw" is to carve away until the skin is exposed, and comparing it to "a thought in mathematics" makes the process feel cold and exacting. Even the payoff is odd: the speaker finds "failures / to build upon," so growth comes from what went wrong rather than what went right.
The images that follow lean toward loss. Something "green as a burial / in the garden" suggests new life and death sharing the same ground, as if the self being made is also being buried. A "lost manuscript / going to a thief" suggests something valuable that is absent and perhaps misused by someone else.
The ending is surprising in its calm. The speaker is "not at all sorry / to be asleep," which could mean relief, numbness, or giving up the effort. The mood is quiet, dry, and a little eerie, with a flat acceptance where you might expect regret.