It's hard. Really hard. And yeah, you can say that willpower is required here, but honestly--if any of us had that kind of willpower we wouldn't have the weight problems that we do. Brain chemistry is a powerful thing, and the more research we see, the more I'm convinced that we're dealing with forces we just don't even begin to understand right now.
That being said, I think I'd try one more time to get the idea across that this isn't like a "regular person" going on a diet and trying to shed a couple of pounds. This is more like an addict trying their best to get and stay clean of drugs--because they know that if they don't, they're going to die. And if an alcoholic/addict lives with a person who's using, it's only a matter of time before they start using again themselves. If your girlfriend remains unsupportive, then you have to make some tough choices. Maybe live on your own where you can control what's in the house.
I'm really sorry you're going through this. It's a hard enough struggle when you have the full support and faith of your life companions--I can't imagine how difficult it must be to do it without that support.