I think this song is about being with someone, but no longer being in-love with that person.
"I have no instruments, I'm using yours" - This is about being so drained and having no energy left in yourself to fight for the relationship. You're basically using up the energy that your partner has left.
"and when I'm home, I'm not at home" - This is talking about how you can be with that person and be in the same room with them, but feeling completely detached like you're not even there. Your mind and your heart are a million miles away.
I think this song is about being with someone, but no longer being in-love with that person. "I have no instruments, I'm using yours" - This is about being so drained and having no energy left in yourself to fight for the relationship. You're basically using up the energy that your partner has left. "and when I'm home, I'm not at home" - This is talking about how you can be with that person and be in the same room with them, but feeling completely detached like you're not even there. Your mind and your heart are a million miles away.