My girlfriend keeps saying we should eat out, and my diet keeps getting ruined.

I'm serious this time, I've decided to lose weight, but my boyfriend keeps coming over every evening with hamburgers and pizza. I say no, but I just can't resist the smell when he's eating, and I end up throwing my whole diet out the window, telling myself it won't hurt to have just one bite. I mean, shouldn't a partner support you instead of being a hindrance? Do you think I'm exaggerating, or is this really an annoying situation?