To be honest it sounds as if you've created your own monster. Picky dogs are created by their owners. Typically the more we 'cater' to our pets, the more they demand of us. They know we'll give in and try something new, so they turn up their nose each time they're bored of something.

She's got YOU trained pretty well! My advice is to find something that she liked in the past, did really well on, and only give that to her. Leave the food down for a while and if she doesn't eat it within a certain amount of time, remove it, and try again the next day. Some dogs will be really stubborn and go two weeks without food, but they'll never starve themselves to death.

I've had dogs turn up their noses in the past. The only time I go out and try something new is if it's show season and I need them to keep up their weight. Otherwise they can be as skinny as they want to be. It only lasts a few months, and once they realise that nothing new is coming, they give in and start eating normally and keep the weight on as per usual.
Some dogs just aren't big eaters. Nothing wrong with that; Dogs who are slightly "underweight" (in today's standards) tend to live longer and have a much lower rate of cancer. If she wants to keep herself lean and trim, I'd let her.