So you want to start acting like your girl doesn't have you so she goes "oh man, something's going wrong in the relationship, I better work hard or else I'm getting dumped?" IMHO, that sounds like emotional manipulation and the complete wrong way to keep a girl. Keep her happy that is.
Don't get me wrong, I get what you're wanting. A guy who can just walk away from a situation he isn't happy with is infinitely more attracted than someone who can't. But that doesn't mean you can't be happy with a situation and show it. I'm more than willing to tell a girl that I'm hers. But she knows at the same time that I'm not going to take any **** and if I get more than what I think is appropriate - I'm out of there.
Have you been acting like she does have you? If you suddenly flip-flop and get all "I'm no-one's girl" then she is going to freak. Do you want her to be your girl? I think there comes a point where, if you want the girl to be your girl, you've got to go, "your my girl, and I'm your man...now go down on me!"