I think it's pretty common to be tired and in a general lower mood than normal after you've been dieting for a while. I know that I can't think or do much other than sleep while I am trying to diet. I used to be the same way as you while I was being healthier: full of energy and happy. Things change, I guess, and the body just doesn't like it after a certain point. That's why I have had to up my caffeine intake by a huge margin now that I'm at the lowest body fat percentage that I've ever achieved.
