While Juice Fasting you don't need protein. Your body, once ketosis hits, will start to eat it's own protein stores. Starting first with the damaged/dying cells which all contain protein.
There's a Science invloved in fasting. It's not like regular eating but a whole other process. Your body shifts gears, per se. Your mind won't starve, in fact, you'll actually start thinking more clear than when you weren't fasting.
Do you have a good resource book to help you through?