Well, something my husband did for me when I was skinny and retarded was asking me a couple times a day if I was staying on track with my calorie goals. I also tracked my calories on a whiteboard in our shared office, so we could both see it. If your gf is willing to participate in a similar system, maybe you could write down the calorie content of your meals on a whiteboard after you eat them, and having that extra layer of accountability could help you resist overeating (don't let it turn into her having to micromanage you, though).
Also, if she knows that certain snacks or foods particularly tempt you to overindulge, she might consider not keeping them in the house while you're on a diet, or keeping them in limited quantities. If she likes to cook, she could also help you with meal planning or finding new low-calorie recipes to try.
ETA: Forgot to add in my last post that, when calculating your daily calorie intake, you should shoot for moderate weight loss rather than slow or fast. Slow will be too slow and you'll get discouraged at the lack of visible progress, but if you try to lose weight too fast you'll get hungry and angry and it might fuck with your metabolism.