Dude, I can totally relate to where you're coming from. My situation parallels your in many regards. I've never dated a girl that I've felt lucky to be with. Each of my girlfriends has had certain major issues. None of them has been incredibly stunning. I once went out for two dates with a gorgeous 9 and had a shot but screwed up due to inexperience.
Anyway, I have two thoughts. There is absolutely nothing wrong with wanting to be with an incredibly physically attractive woman. That desire is hard-wired in your genes as a man. However, due to the rise in obesity (now over 40% of women are overweight) and the generally bitchiness of girls in America, there is a decreased pool of attractive women available. Therefore, the average man now has to bring about twice as much game to attract a reasonably attractive woman as he would have 40-50 years ago.
In the end you basically have three options:
Balls of Steel - Work out religiously, craft your life to be awesome, approach every gorgeous woman you can...etc.
Requirements: Dedication, lots of work, tons of raw testosterone.
Reward: Dating much hotter women
Settle - Accept your current level of attractiveness along with the kind of girls you can date, and try to enjoy it.
Opt Out - Determine that dating in the modern world isn't worth it and spend your time, money and energy doing things you love.
Regarding your girlfriend's question, the short answer is that you should never give her a direct answer. You're in a good place if she perceives you as more attractive than her. She's lucky to have you and she just wants some emotional validation. Best response: "You'll need to take off your clothes so that I can determine how attractive you are" (w/ a smirk and a wink). Other acceptable responses include humorous teasing deflection, or mentioning something about her that you genuinely value and appreciate.
The best response is to go and actually be dating a gorgeous woman. Are you willing to do whatever it takes to make that happen?
i do like models and generically pretty women.
i've had women tell me that i will never land such a women.