Studying memory issues fasting

I hear someone people say they get mental clarity fasting & others say they feel horrible.

Ik about self electrolytes but does anyone know WHY else some people have different experiences.

I done a long fast (2 weeks then a day of eating and another 2 weeks) but I felt terrible, u was trying to read stuff for uni & I honestly may have well have been reading another language, nothing sunk in.

Aside from the fact my mind can't seem to fast again even 1 month later I really want to, so just wondering if there's any tips. I've considered creatinine as I'm low in it but not sure if that ruins a fast, I'm not fussy about it being 100% No cals ans onky water etc but I'm just not sure what to do