In addition to the themes of elegy and imperfect love mentioned in the previous answers from other educators here, there is possibly a theme of existentialism at work in this poem. An existentialist view generally accepts that life is without meaning or purpose, that things happen for unexplained but banal (rather than mysterious or mystical) reasons.
Using this interpretation, once can see that there is very little in the sonnet that is not based upon the real and final condition of death. The word "darkness" implies a state of nothingness and loss of consciousness after death. The word "corruption" is a reference to the earthly decay of the body after death, and therefore the idea of what occurs after death is focused on those left behind (the poem's intended audience), and not the fate/future of the poet.
And yet, being a thinker and artist, the poet's words here are bound up in ideas of ego and remembrance, perhaps partly because of her body of work and its legacy that she hopes will live on. The sonnet's language makes no explicit reference to this, only a somewhat vague description of "a vestige of the thoughts that once I had."