I'm not a doctor, so I can't answer a lot of your question, but I can tell you that juicing has helped a lot of people, and they definitely have lived off (juiced) fruits and veggies for months and probably decades when eating them fresh.
Dr. Schulze has (or had last time I checked) a program called the save your life program. He also had a 30-day cleanse program. My dh-at-the-time and I got the save your life program because we wanted a strong cleanse and it really did help both of us.
So, while I can't allay your fears in regards to how bad you feel, I can tell you that drinking juice from freshly juiced (preferably organic) fruits and vegetables should not be harmful at all, and should be helpful. And, I'd recommend Dr. Schulze 's products (herbdoc(dot)com) because they give more nourishment than juicing alone.