I'm not sure if this is correct, but part of astral projecting is completely identifying your conciousness with your energy, to insert your conciousness in your energy and make yourself (energy) leave your body? Maybe that's why it's so dificult to do it, if you're scared of your physical well being, ainxious or whatever it might block you from leaving your body(?). Emotions or physical needs that involves your atention to your body will mane your exit(?)