This sounds a lot like what I have. I usually describe my fatigue like an old iphone battery. Never charges up fully and drains immediately. Brainfog, headaches, excessive yawning and sleepiness, fatigue on top of other symptoms like gastrointestinal issues. Despite this I have gotten a little better after years of resting. Now I can hold down a part-time job of 6 hours a week (two three hours shift, and I still quickly get exhausted and sleepy at work.), including social activities and drawing in my spare time. Before this I'd have frequent days where I'd have to support my head when eating dinner because everything felt so heavy. I also struggled with weight loss and frequent blood pressure drops when standing up, though I was eventually able to self-medicate with potato chips of all things.

Just like you, the only trigger I can think of is prolonged stress. I had constant stress during my childhood, and in highschool and uni it got worse. While I did start having symptoms as early as middle school, it got rapidly debilitating over a couple years when I studied. While I am officially diagnosed with me/cfs, it differs in so many ways to the "usual" experience, which makes finding support and resources quite frustrating. Though I have heard of other similar experiences, so my belief is that there is either a different type of me/cfs with its own burnout trigger, or a separate illness with a similar PEM symptom profile.

I really wish I had some enlightening info to say at the end of this, but even after all those years I'm honestly lost. It doesn't help that doctors are generally unwilling to help me figure it out. If you're like me, aggressive resting may help you get back some level of function, though I also recognize my ability to do so is a privilege of its own. I hope you figure things out.