Sorry it doesn't work for you. But at least you did lose some weight though. Good luck joining the gym. I read somewhere we humans are programmed to get used to a particular habit once we do something daily for at least 21 days. Hopefully you won't quit going to the gym too before about a month.

You sound like you've been super dehydrated.

But anyway, Keto diet or not, if your wish is to lose weight, then you must watch what you are eating. Sugary stuff must be avoided. 80% of our weight is associated to our food intake. And with additional support from your gym regime, it will help burn the fats faster and live healthier. Good luck!

if you're constantly tired while on keto (and on the 3rd week even), perhaps your body did not adapt to it.

From my experience, I've seen people who "claimed" they're doing keto, but they're essentially just on low carb diet (big difference there). majority of people totally underestimated the amount of carbs you can consume while on keto. If you're just eyeballing your food and not knowing the macros (carbs, carb to fiber ratio), then most likely you're not even close to getting keto adapted.

But if you did everything correctly, then your body may needed more time as from what I've read, some people make take even up to 4-5 weeks before getting keto adapted.