This mirrors what I experienced when I went through a severe burnout 2 years ago. I had some health issues like sleep apnea and digestive problems converge before I knew about them, so even though I was working out heavily 5 times per week, I was fat and unhealthy and had lost my resilience.

My work put us on call at night 1 week out of 3 to monitor an ailing server and my productivity fell precipitously almost immediately. That was the last straw, so even though I thought I had the discipline to handle it, my body thought otherwise and I started crashing (falling asleep at my desk, having so much brain fog that I struggled with basic chores, etc). I miss that job often, but the writing was on the wall at least a year before that when I noticed that nobody listened to my expertise, they just wanted output. That's the surest route to burnout that I know of so far.

I did have a lot of concussions in elementary school, but all I remember is the nausea and laying in bed for days. Severe burnout/depression feels similar in that one loses their executive function. To me, burnout felt like what I imagine having a stroke might feel like. My problem-solving faculties shut down, so it was like my mental muscle memory of everything I had learned was no longer there so I was consciously aware of every single step I had to perform (which left me frequently overwhelmed). It also felt like maybe the left and right sides of my brain weren't talking. When I thought about doing any task at all, no matter how minor, I felt low grade pain over my entire body like the feeling you get as you walk slowly into a cold body of water. Where there used to be excitement at the anticipation of doing things I enjoyed, there was now only a void of apprehension and dread. Everything felt like work, even recreation.

I had to start from the beginning and relearn how to write todo lists, journal about what I wanted out of life, address all of the physical issues I could, etc etc etc. The self-help stuff is all true. The only part they are missing is that IMHO as much as 80% of our mood is derived from physical sources. The science of vegetarianism isn't quite there yet. So for example, I was eating more beans (legumes) and nightshades each day than most people probably eat all week. The anti-nutrients had lowered my absorption of iron, zinc and I'm pretty sure iodine, not to mention that I wasn't taking enough of the basics like B vitamins. Each time I reintroduced traditional foods like bacon, I received outsized returns because of things like tryptophan turning into serotonin. Nutrition is huge.

Anyway, when it's all said and done, the best treatment for me was to slow down and emphasize things like how priceless consciousness is (dignity), and deemphasize artificial human constructs like worth/productivity (materialism). Also I learned that habits take about 2 weeks to form, so it was helpful to list 1 or 2 things I was struggling with, then perform them manually and dispassionately until the habit stuck. Negative self-talk is huge too. I adopted a continuous mindfulness as a set of rules so that whenever my inner monologue is critical, I note it and file it away without engaging. Over time, the positive thoughts grew as my inner child learned that positive reinforcement works.

I'll stop there since I could talk about this literally forever, but I highly recommend talking to someone and seeing a doctor or nutritionist and trying some integrative and holistic approaches. Don't lose years like I did.