OED

Line 53: “Confounded though immortal: but his doom
Reserved him to more wrath; for now the thought
Both of lost happiness and lasting pain
Torments him…”

*Discomfited, abashed, put to shame, or mental confusion; confused disordered, etc.
*Applied by way of execration to any person or thing strongly objected to.

Both definitions fit well into this passage. The person that is being talked about is going through some sort of pain and he is being tormented. The first definition is about being discontent and shamed and this could represent the character in the poem. The second definition is a curse on someone or something, and it is possible that the torment the character is going through is from a curse of some sort.