"And now you are my favourite sin"- could mean I am indulging in you so much that it is considered sinful? And I like it?
"Cause I'm either on your side or you're a mile away"- Maybe it is just a way of expressing in a exaggerated way, either you are very close to me one moment, the other moment I feel you are so far away emotionally?
I must be getting old- songs lyrics don't mean much to me anymore...