The speaker is stuck in a loop of memory and despair. Looking "forward into the past" is a neat contradiction: instead of moving ahead, the mind keeps circling back to painful events that refuse to fade. Sleep brings no rest, and the speaker wonders whether the torment is partly deserved ("some pain I earned") and whether the only prison is their own mind.
As the poem goes on, the feeling of numbness grows. Food loses taste and smell, colors blur together, and each new day brings a chill rather than hope. The speaker avoids mirrors and says they are losing their sanity, which suggests deep self-loathing and exhaustion. Even the big ideas of life, love, death and fear feel muddled.
The final couplet leaves the speaker as little more than a body walking aimlessly. The mood is bleak, heavy and hopeless, and the steady rhymes make it feel like a sad chant that keeps repeating.