I think this song is about holding on to something that you know you should let go of. Whether they left you or you left them, it's hard to let go of something you've always wanted. It's when every part of you wants to rip up his letters and burn his clothes... but then you find yourself sitting on your bedroom floor in his jacket re-reading it.
"Strong enough to leave you, but weak enough to need you." You'd go back to him in a heartbeat even when you know you shouldn't.
You keep everything just in case he ever comes running back. Just in case he changes his mind, you still have everything, including your feelings for him.
I think this song is about holding on to something that you know you should let go of. Whether they left you or you left them, it's hard to let go of something you've always wanted. It's when every part of you wants to rip up his letters and burn his clothes... but then you find yourself sitting on your bedroom floor in his jacket re-reading it. "Strong enough to leave you, but weak enough to need you." You'd go back to him in a heartbeat even when you know you shouldn't. You keep everything just in case he ever comes running back. Just in case he changes his mind, you still have everything, including your feelings for him.