Tongue is white - that is a sign of an excess of yeast in the body.
You suspect that you are experiencing Keto Flu, actually you are experiencing a massive die off of yeast in the body and skin. That is why you feel better as soon as sugar is digested.
Google is your new friend on this one, candida.
Apple Cider Vinegar will help to clear out the yeast, expect lots of gas and sudden weight loss with the die off.
The sooner you clear the body of yeast, the sooner you’ll start to recognize benefits from Keto.