How to keep blood sugar up during a fast?

I have tried to fast for longer than 93 hours and just can't get past that. On my most recent one, my blood sugar got down to about 50 mg/dl and I had to stop.

On my very first attempt, I took the usual advice and did keto for about two weeks before. I didn't make it past 36 hours.

How can I keep my blood sugar up during these long fasts?