I'm in a somewhat similar situation to you, but replace the 3 years with 10 years. I had many periods of 'minor' burnout along the way that I ignored or ploughed through, which in hindsight was a pretty big mistake.
Around August last year I just couldn't continue. I wasn't sleeping, I was frequently run down, and I was self-medicating more and more with drugs and alcohol. It eventually got to the point where simply opening my laptop would elicit a fight or flight response.
I was lucky enough to be in a secure enough financial situation to largely take 6 months off. If you're in a position to do this, I highly recommend it.
I uninstalled gmail, slack, etc. from my phone. I considered getting a dumb phone, but settled for turning off push notifications for everything instead. I went away with my girlfriend for a week and left all my tech at home except for my kindle (literally the first time I've been disconnected for more than a couple of days in probably 20 years). I exercised as much as possible and spent time in nature going for walks, etc.
I've been back at it part time for the last few months. Gradually I felt the feelings of burnout being replaced with feelings of boredom, which is hopefully my brain's way of saying that it's starting to repair itself and ready to slowly return to work.
I'm still nowhere near back to peak productivity, but I'm starting to come to terms with the fact that I may never get back there. I'm 36 and probably would have dropped dead of overwork by 50 if I kept up the tempo of the last 10 years anyway.
I'm not 'cured' by any means, but I believe things are slowly getting better.
My advice to you is to be kind and patient with yourself. Try not to stress about not having a side-project, and instead just focus on self-care for a while. Someone posted this on HN a few weeks back and it really hit close to home for me: http://www.robinhobb.com/blog/posts/38429
Anyway. Five years on, haven’t worked since other than very lightweight consultancy. Live in the woods.
Still wake up at 0430 every morning in a panic. Still grind my teeth. Still flinch every time I hear my phone. Incapable of being kind to myself.
There’s a point of no return, beyond where the brain damage is irreparable. You can learn to live with it, but you can’t ever get rid of it.
I’m damaged, but functional - managed to build a house with my own hands this spring, while writing an ISO27k1 ISMS, while living off grid - I don’t just sit around on the couch - hell, don’t have a couch.
So yeah. Coping rather than being cured is the best many can hope for - and I’m just about coping.
I hope you find a path forward where you have health and happiness.
It does get better - consciously, I’m a much happier and calmer person than I was - but whatever part of me it is that screams in their sleep and kicks me out of bed in the pre-dawn hasn’t improved one jot. I wish it would.
And yeah - very powerful piece of writing. She's a published fantasy author and I'm going to try one of her novels (https://www.goodreads.com/book/show/77197.Assassin_s_Apprent...)
I think my biggest since issue was - and probably continues to be - very poor work/life balance (no hobbies, basically no social life outside of work, etc). Obviously it's hard to disentangle cause and effect, but I suspect a prerequisite for getting burnout is having a mania or hyper-fixation on work.
Of course I'd expect someone already burning out to deceive themselves and game their own metrics (like reading and answering emails at night and not counting the time spent).