Sounds like not enough electrolytes. Wiki has recommended amounts of sodium, potassium, magnesium. Two teaspoons of salt doesn’t sound like it cuts it. If you start feeling worse I’d suggest considering cutting the fast short to be safe.
I’ve never been able to get past the shit stage myself. The longest I’ve gone is 10 days (once), but usually by day 2 or 3, I am freezing, which I can deal with. But then, on day 4, my heart starts racing, head aches, and I get lightheaded and can’t sleep. I’ve gone through this 20-30 times, and I’ve not found any combination or amount of electrolytes that staves it off. If I try to push through it, it just gets worse. Usually, my lack of ability to be productive or my heart racing at 120 bpm in bed at night will cause me to end the fast on or around day 4. But damn, if I wouldn’t love to do 20 days.