Yes, as you know the speaker goes into a distressed mental state, and the only tension is that he slips into numbness state as if a person feels after ingesting an opiate. “Lethe-wards” refers and an allusion to a river in Hades, an underworld in Greek mythology. Here the meaning of “lethe” is forgetfulness. Thus, he feels numbness to forgetfulness and in anguish that he wishes for forgetfulness.
Allusion in “Ode to Nightingale”