I fasted last fall and had the same experience. Very very weak. I was also going off of coffee and had adrenal fatigue. I could only last 2 days. I got scared that I was losing too much nutrition. So I did intermittent fasting but did not stick to a strict schedule. I basically fasted 2 -3 days a week. It really worked for me and I got rid of my low blood Sugar issues. When my family members saw me after doing this for a month they said I looked beautiful! :)
To clarify on my eating days I ate very lightly - Watermelon and other fruits, whatever my body was craving and a few times I had sushimi.