The writer has an ambivalent feeling about love (the 'evils' he refers to in his parenthetical remark).

A contemporary writer would most likely include the word 'like' in that description (it is only implied in Shelley's text), and might also change the order of its elements to make the intended meaning better apparent:

solitary and joyless remedy for evils which, to the memory, seem like blessings.

The narrator has an ambivalent feeling about love (the 'evils' he refers to in his parenthetical remark).

A contemporary writer would most likely include the word 'like' in that description (it is only implied in Shelley's text), and might also change the order of its elements to make the intended meaning better apparent:

solitary and joyless remedy for evils which, to the memory, seem like blessings.