My interpretation of this song is that there's a person who you have a history with and who is definitely not a good idea - either the circumstances within you get with them are bad, or they mess you around, but you keep going back for more and can't keep away from them. And every time you go back to them you're essentially losing your innocence because being with them is wrong (as I said, either morally or even just for your emotional wellbeing) and the losing the halo part I take as hooking up with the person ("knew I couldn't help myself")
My interpretation of this song is that there's a person who you have a history with and who is definitely not a good idea - either the circumstances within you get with them are bad, or they mess you around, but you keep going back for more and can't keep away from them. And every time you go back to them you're essentially losing your innocence because being with them is wrong (as I said, either morally or even just for your emotional wellbeing) and the losing the halo part I take as hooking up with the person ("knew I couldn't help myself")