I know all about the feeling weak with no energy. I'm on day 22 of a full liquid diet. My surgery date is April 8th. I have to assure you that the feeling hungry and weak will pass. Its not a good feeling. I quickly learned that water was the solution to the headaches I was getting. If I feel one starting I relax drink water and usually start feeling much better, quickly. I have also found that Special K has K2O water mixes that work well in controlling hunger. I've lost 21 pound in 3weeks with this liquid diet and walking. Best wishes to you and keep me posted on your progress.
Cheers!
-M