I'm really happy now, but I understand burn-out at a much deeper level than I had before. There was a time when I would think "just get over it," but now I understand it has very real effects. When you don't want to think, it's not always a simple case of laziness. It takes work to switch into your "system 2" (as called by "Thinking Fast and Slow") -- your focused mindset. Then once you're in this mindset, it's no picnic.
What's fascinating is that if you stay in system 2 for too long, you begin to show signs of paranoia and depression. You become withdrawn and isolate yourself further from society. You might hypothesize this is a natural reaction of an _injured_ animal, but it's hard for you to understand that your brain is exhausted and needs to rest. Instead, you say "go faster!" and it begins to try to come up with more & more schemes to prevent you from going into system 2. Fantasies that would have meant nothing to you well-rested are now of utmost importance because you're actually procrastinating because you're actually fatigued.
Burn-out is a real thing. I love work and I work very hard, but there's times when you have to say "You know what, this weekend is all mine." Those are the weekends I sleep anywhere from 10-14 hours each night, and not because I'm indulging, but just like muscles, my brain is resting and hopefully "growing".