Ketones don't fuel your body or brain entirely. You create new glucose from protein on ketosis, enough to survive (good macros) or too much that kicks you back out of ketosis (too much protein).

You can never drop glucose in the body to zero.

Also, entering full ketosis can take a few days so you were likely feeling the effects of low glucose while not in ketosis.

It takes dedication to get into and stay in ketosis, it's definitely not for everyone. But if you do stick with it, you definitely should feel a lot more energetic.

The first time I entered full ketosis I had too much energy to sleep for a week.