Hi Bumblebee23,
I totally understand your situation because fatigue, weakness and tiredness dominated in my water fasts. They are quite common side effects of ketosis, in which metabolic state you are right now. On the other hand, some people in ketosis often feel a renewed sense of well being because ketone bodies calm down the brain cells. All depends on the adaptability of the faster's organism to the ketosis.
You are still lucky as headache, nausea and dissiness are even worse and not so rare side effects. If you can afford to sleep, do it without any hesitation as sleep could be blessing in your situation. Lets hope that your weakness will soon disappear so you could enjoy the tranquility and heavenly feeling that ketosis ought to produce. Happy fasting!
Anne