Considering my girlfriend recently broke it off with me for the same reason, I can easily relate to this interpretation. The narrator feels bad about the way he feels: "thirst for substance somehow isn't right," "it's killing me inside," but he also feels bad about further leading her on: "I don't wanna be by your side, if something isn't right." Even though he is unfulfilled with his relationship, it is obvious he cares a lot about his significant other: "please take these feelings for her inside," she probably treats him very well. But he is determined to end it soon: "this is our last goodnight," and preparing for it is killing him: "my chest hurts when I breathe tonight." He knows she has a right to be upset with him and thus will respect what she has to say: "say what you will, say all that you can." I even feel like the lyrics "because your heart seems so cold tonight", and "how could you watch innocence forgone," are directed to himself.
But that is just my interpretation. Alcohol/drug abuse makes sense too.
Both the words "thirst" and "substance" have several different meanings. "Substance" can also refer to matter; think "substantially". Perhaps the narrator is suggesting his relationship is lacking substance. Thus he desires (thirst) a more meaningful relationship, one that does not feel so "blasé".
Considering my girlfriend recently broke it off with me for the same reason, I can easily relate to this interpretation. The narrator feels bad about the way he feels: "thirst for substance somehow isn't right," "it's killing me inside," but he also feels bad about further leading her on: "I don't wanna be by your side, if something isn't right." Even though he is unfulfilled with his relationship, it is obvious he cares a lot about his significant other: "please take these feelings for her inside," she probably treats him very well. But he is determined to end it soon: "this is our last goodnight," and preparing for it is killing him: "my chest hurts when I breathe tonight." He knows she has a right to be upset with him and thus will respect what she has to say: "say what you will, say all that you can." I even feel like the lyrics "because your heart seems so cold tonight", and "how could you watch innocence forgone," are directed to himself.
But that is just my interpretation. Alcohol/drug abuse makes sense too.