The speaker in the poem is trying to distract himself from his sorrow. He uses books to aid in the distraction. He says, "vainly I had sought to borrow / From my books surcease of sorrow" and here "surcease" means to stop or "cease." His loss of Lenore has left him feeling deeply saddened. It is her absence that he tries to avoid, but can not help but focus on. She is absent and he is therefore faced with the bereft feeling of nothingness. This is why Poe repeats the word "nothing." He is left with "Darkness there and nothing more."
The speaker even calls out Lenore's name into the darkness, as if to fill the nothingness with her name. But, of course he gets no answer; just more nothingness. He tries to forget Lenore but he can not. He cries out to quaff (drink) "this kind nepenthe and forget this lost Lenore!" Nepenthe causes forgetfulness. But the raven replies "Nevermore" meaning he (the speaker) can never forget her. He will therefore always be pining for her and always be sad that she is gone.
The raven represents death. His very presence represents the death of Lenore and reminds the speaker of his own mortality. These are two things the speaker wishes to forget, but he can not. The speaker is so psychologically distraught that, even though he tries to forget, it is he who continues to engage the raven. He keeps asking about Lenore, if he will see her again, if his anguish will ever end. Since the raven only says "nevermore," the speaker continually sets himself up to be dismayed. The speaker wants his sorrow to be over but he continues to torture himself in this way.
In the last stanza, the speaker notes how the raven is still there. The raven representing darkness and death is perched on top of the bust of Pallas (Athena) which represents truth and reason. The speaker is in such a dark, morbid state of grief that he is allowing this symbol of death to override his own ability to use logic and reason. If the speaker continues to engage the raven, he will continue to dwell in his sorrow. If he continues to be consumed with thoughts of nothingness and the idea that he will be "never" be happy, he will never come out of his depression.