At the incredible risk of being flamed, I personally believe this song is about negative emotions that put you through hell one day after the next, but through those negative emotions comes an acceptance and determination to strive as a human being. The line "Shit adds up at the bottom" applies to all the shit that goes on in your life, while the line "I have swallowed the poison you feed me, but I survive on the poison you feed me" states how your weakness, failure, and negativity helps you become a better human being - even if it impacts you in a negative way, it pushes you through life, the hard way.
So, am I a good person with an amazing taste in interpretations, or am I a clueless retard who needs to pull his head out of his ass?
