I have a 21-month old son who is going through the exact same thing...since about 18 months or so. I even spoke with his doctor, who said it is normal, and that he will get what he needs on his own. She did do a test to make sure he is getting plenty of iron, but otherwise, didn't seem concerned. There are many days that I wonder how he continues to grow and thrive on so little food! My doctor suggested that I may be giving him too much milk before meals, but I don't think that is it. He just doesn't eat much!
He is definitely a grazer, and his favorite way to eat is when I cut up lots of small, healthy snacks and arrange them in a muffin tin. He will carry that pan around all day, slowly snacking on the fruit, cheese and grain-products that I fill it with.
I totally understand how you feel...lots of wasted food, and time! I would say to just keep providing healthy foods and she will take what she needs. Maybe even try the muffin tin method (which I got from Dr. Sears)! Good luck!