2D (Stewart) is feeling depressed and useless for reasons explained above. He is saying he's useless as in he has no use for himself and is a sign of self hatred, the second part of the line "but not for long" indicates that he will end his life soon because of his depression.
The speaker, 2D (or stewart pot, stew pot for short, which means weed) has mixed feelings and is confused, because he was using drugs, explained in the next line.