I think the "all in my mind" part is a reference to all the things being sung about. It's his imagination running wild. Because he doesn't approach the girl. He just assumes a bunch of negative things that could happen if they were to come into contact.
I think the "all in my mind" part is a reference to all the things being sung about. It's his imagination running wild. Because he doesn't approach the girl. He just assumes a bunch of negative things that could happen if they were to come into contact.