Most important part is be honest to yourself, dont compare to others (those are your goals and they dont get better or worse if others reach better things etc.) and listen to what your body tells you, besides hunger maybe ;)
Health > Diet.
Feeling weak is most of the time normal, i feel nauseos every 2nd day...
Especially if feel really shakey, lose sense of balance etc. eat something.
And dont forget to drink ALOT, you will lose alot of water and you need to refuel, also your body goes trough detox like state where your organs are getting cleaned (you propably have very yellowish pee the first 3-4 days).
Also if you eat something, try to eat very healthy... fruits with very low sugar(blueberries are my favorite, nothing beats them), vegetables, meat with low fat(kangoroo, chicken etc.), fish (even with high fat), eggs... if you eat stuff like that you will have much energy with very few calories and your body will thank you.
Add Salt (also use quality salt f.e. seasalt or himalaya salt) and Lemon Juice to your water, only a pinch of salt, otherwise it would make drinking water useless lol :D
The lemon counters acids in your body, sounds stupid but its true, maybe its not called acids... my english is not too good :D
Drink at LEAST 2 Liters a day, drink as much as you can (you shouldnt go above 8 liters or so...
I think its little above 10-12 liters where you could die of water poisoning, your body would just try to get the water out of the system and the water would take all the good shit with it.)
Also dont drink 3 Liters in the morning and call it a day :D your stomache has to get smaller and water widens it, so just drink like a glass of water every 1-2 hours.
Allways keep something to drink with you so you dont forget.
What also helped me is supplements, a cheap one that maybe helped me (i felt better but could also be other factors) is L-Tryptophan, basically what it does is, it slightly increases your mood over day, and you can sleep better at night.
Treat your body like its the most important thing in your life, because it actually is.
Your body determines how long you life, how happy you are or how happy you can get, it determines how others look at you (even tho we shouldnt care ;) but i guess everyone does at least a little) .