Who're you going to take fitness advice from the guy who is eating right and working towards a six-pack, or the guy who eats right and has the six-pack?
@Charnell hits the point right on the head.
Imagine this is your girlfriend:
She's not a super model, but she's pretty, has an average body, and is doing ok. She's happy with where she's at.
Now, the guy on the left is you currently:
He's fat. Out of shape. A physical embarrassment.
Now, imagine the girl above coming home from the gym and the fat guy badgering her: "Babe, I know you want me to do the whole gym thing, but I finally lost 5 pounds. You should really quit your gym too. This is the only real way to lose weight. You stay at home and just workout here. What you're doing is pretty stupid. And no, I don't want to go the gym. That's pointless since I'll never get into peak shape like I would doing my own thing."
He continues this spiel for a YEAR AND A HALF, before losing another 3 pounds. It takes him another 4 years to look like the guy on the right - still not that attractive but finally in the shape he would've been if he went to the gym.
My point is: The only way to convince your girlfriend of the fastlane is to actually be fastlane. Instead of trying to convince her that what you're doing is right, just be grateful that you have a girlfriend that hasn't left you yet after a year and a half of you making no money and staying at home.