the longest fast I've managed is 12 days, I was trying to go for 21 but my body fat got too low and the hunger was too much.
I drank 2 litres of water a day which was a mistake, messed me up a bit probably.
The hardest part was the refeed, I found it really hard to control my hunger and ate too much junk.
My advice to you would be to start off doing short fasts and gradually build them up. Like fasting from Friday night to Monday morning.
Feeling dizzy if probably because you're doing too much exercise, you should not be doing any apart from 10 mins gentle in the morning to get the lymph flowing.
Try to drink as little water as possible.