You're not eating enough. For most people, if you get under 1400 calories, your body jumps into starvation mode and conserves energy (burning fewer calories). Try adding some high-quality fat to your diet, such as a tablespoon of coconut oil or an avocado each day.
Starving yourself creates a horrible circuit -- your body needs fewer calories to survive, so you have to eat less, which makes your body conserve more, so you need less, etc.
Go back as close to nature as possible. As many veggies as you can stand, good quality (not processed) meats. As few processed items as possible. You might also, for a time, cut out dairy and wheat to see if you're reacting to them.