As wonderfully beneficial and healing Water Fasting is to the body there are certain conditions that warrant not fasting. Some people have to be careful. Your body seems to be speaking to you clearly to perhaps reconsider other options or make sure you don't have a predisposed condition that makes Water Fasting unwise. I find it's always a good idea to listen to one's bodily reactions, no matter what expert opinion dictates. Nonetheless, I found this information notated below on Dr. Ben Kim's website. You may find it helpful. I'm not sure if you fall into this category, but notice one of the symptoms is not being able to complete a fast of a longer duration. Perhaps arrange a physical exam to make sure everything is in good order and there's nothing out of sorts with your blood or other systems. This is not to alarm you, merely to bring it to your awareness. I'm sure you will also get lots of good tips from other fasters on the subject.
"Q. Can anyone fast?
A. There are a handful of exceptional circumstances in which it is not advisable to fast. A small portion of the population has an inborn error of metabolism whereby they lack an enzyme that is needed to process fatty acids. Since fatty acids are needed as an alternate source of energy during a fast, it would not be safe for such a person to pursue a fast of significant duration. This disorder can be recognized early in the fasting process by a trained observer.
Intake of certain medications, certain liver and kidney disorders, states of extreme weakness or malnutrition, pregnancy, and certain types and stages of cancer are other examples of conditions that are not conducive to fasting." -Dr. Ben Kim
I agree that perhaps you should try a juice fast. But maybe before that consider simply modifying your diet. Maybe you're deficient in a mineral or two. Most of us are until we find our own rhythm and balance with our health goals and nutrition plan.