This is NOT simply a song about unreciprocated love... I personally Believe that it is a song about being in love with a person who is destroying themself through drug addiction.

The first verse is about coming to terms with the fact that the person that the persona loves is destroying themself.

In the sencond, the line "all I have is one last chance" highlights the fact that the persona has little, or no time to save the person from their addiction. He is unwilling to allow them kill themself with their addiction (I won't turn my back on you"), but he fears that he cannot save them from it ("and I can't save what's left of you").

The chorus further illustrates the persona's struggle against his love's addiction. The lines "sing something new, I have nothing left" describe how the persona is sick of hearing the same excuses and lies about the addiction, and how he is at the end of his tether. He also admits that he doesn't want to live without the said love interest, but she is unable to quit ("the fighting never ends, I can't face the dark without you").

The persona then continues to describe how his love is not who she used to be, and how she is destroying herself in the lines "come back down, save yourself, I can't find my way to you." He also describes how he feels powerless to save her, and his reluctance to accept what is happening; "and I can't bear to face the truth" I wanted to forgive, I'm trying to forgive, don't leave me here again."

The final verse shows how the persona finally gives up, despite the fact that he still loves the person killing themself with addiction, and that although he does love her, he has to leave and forget her, because he is powerless over the situation.

This is my interpretation of the song, but it is not correct, like all thing that are subjective, so I'd love to see some more.