Re: 'People talk about these "wondrous" effects of fasting. All I got was headaches, tiredness, feeling sick and constantly thinking about food.'
It sounds like you went into it without understanding the processes that will be unfolding - you didn't do enough research, otherwise you'd understand the symptoms you described.
The first time you fast will always be the roughest - most challenging too, especially since you've likely been eating highly inflammatory foods your whole life, likely without any break - and so by removing those foods and that inflammation from your body and brain, your brain is going to start functioning differently - and better.
Your brain 1) has to prune the pathways that were dependant on such a high level of sugar in your system (and not running off of ketones), and 2) inflammation has a depressant effect on the nervous system - and so you're essentially self-medicating to depress yourself when eating inflammatory food - and when you remove that, that depression is going to life, so your brain is going to want to start working faster as well - and so there will be pathways that will want to open up again that are no longer being suppressed-depressed.
It's also possible you weren't drinking enough water - or something was off with the electrolytes amount; you don't need any electrolytes or supplements if doing 3 days or less.
Refeeding syndrome is also a myth or rather it's a concept that formed from misunderstanding and poor guidance: "you have to eat small amounts and reintroduce foods slowly so you don't cause X, Y, Z." Bullshit. If you're eating food that's harming you, that you're sensitive to and your body is sending you pain signals for - but then you ignore that discomfort and continue to eat it, eventually you'll get somewhat "conditioned" to it - so you won't feel it as much - but that pain/discomfort/disruption/signal will still be there.
When I break a fast - I've done many 3 days, a dozen or so 5 days, and a couple ~10 day fasts - I literally cook one or two massive high quality and high fat steaks and devour one immediately - and the second usually within 2 hours. I don't feel any discomfort after, and I can feel my body being happy, and my mood lifts as my body knows it has food again; sometimes I'll have kale and a pack or two of raspberries - but I usually only add that on the following day. Simple, non-processed foods that I know my body is fine with - and the steak and kale being highly nutrient and energy dense (for their food group).
I break my fast with, return to food I know my body is good with because I've done the work to figure out (and not only through an elimination diet but also diagnostics that's far more efficient and provides concrete results to follow) - rather than in an unaware-unexplored fashion ingesting whatever I've been relatively unconsciously been eating for most of my life.
It was only in my early 20s a doctor told me to stop eating wheat due to symptoms he saw I had. No one prior to that ever suggested food could have been a source of any of my problems. Soon after that I realized I was allergic to eggs, learning later it is specifically egg whites - to which my mother also later learned she is also allergic to.
The mind is a powerful thing and will allow us to do things that are harmful to us, even pulling us away from, disconnecting us from feeling, suppressing us from feeling this harm that we're doing to our body - whether from food or physical activity. Most people don't do the work to explore, in part because no one has explained it to them thoroughly enough for them to be motivated enough to try.
I'd urge to you to try fasting again - but also to be gentle with yourself, don't be forceful or violent with yourself. Just start doing 3 days at a time to start. Likewise, figure out what a healthy-clean diet is for you - nd do that diet for at least 3-5 days before starting your water fast so then you have good energy and nutrient reserves at the beginning of the fast. And then break your fast with that same diet. If you need help figuring out how to figure that out, let me know, and I'll do my best to help. I plan to eventually write a book on the topic, at least a chapter of it, but I don't have anything complete to share with you yet in this regard.
I'd also recommend you read this other comment I just made - which will help you better understand processes that occur during fasting: https://news.ycombinator.com/item?id=31409602
And here's a 30 minute video by Dr. Jason Fung explaining by water fasting is good, healthy, and safe for us (arguably unless you're underweight and don't have fat reserves to burn): https://www.youtube.com/watch?v=tIuj-oMN-Fk - Dr. Jason Fung - 'Therapeutic Fasting - Solving the Two-Compartment Problem'