The people in the know now understand that what we call "burnout" is not a psychological disorder at all, but sub-clinical[1] adrenal fatigue caused by stress, period. In other countries with superior medical system integration, like Austria/Germany, this is an accepted medical fact.
If you stress out a rat long enough, its adrenal glands will actually bleed. If you stress out a human long enough, you cause first a huge ongoing spike in cortisol etc., and later, a serious drop as the damage is done. This can be measured easily using a 24-hour cortisol saliva or blood test, but it rarely is. (Related hormones: DHEA, thyroid).
Symptoms of burnout are just like the symptoms of CFS/low thyroid because they are part of the HPA (Hypothalamic-Pituitary-Adrenals) Axis. They're tightly coupled.
While short bursts of endorphins will make you feel better, the effect will be temporary. Endorphins will not make your adrenal glands or thyroid work again. Instead, they mask the symptoms… but only for a while. There are only three ways to get better: Dramatically cut stress. Wait. Take medication. Pick 2.
[1] "Sub-clinical" usually means "not measurable," but in the case of "burnout" and adrenal fatigue, it is absolutely measurable. However, I call it sub-clinical because of the way most doctors think of the adrenal glands. Most doctors believe that the adrenal glands are binary: they are either working perfectly, or totally non-functional (Addison's disease). Unless you're on the verge of death from lack of cortisol, they will tell you you're fine.
If you have burnout, get your 24-hour saliva test. Take your morning temperature with an old-fashioned thermometer under your arm for 10 min. Look up the other symptoms of adrenal fatigue and hypothyroid. There's plenty of research published about how burnout is measurable and related to the HPA axis -- you don't have to take my word for it. There are lots of support groups, too, where the sufferers have often presaged the research.
And for god's sake, stop telling yourself it's about failure or investment or whatever and stop stressing yourself out, period, or you can get like I am and it can become permanent.
EDIT: If you google "burnout syndrome" you will read several sources claiming that there is no proof it's a real thing. This is 100% false. They also say it's not accepted by the medical community, but rather a quacky way to sell supplements. It's true that most doctors are unaware of the research, but the ones who do treat it with real medicine (see below).
It's painfully easy to say "There's no proof" but a 5-minute search of Google Scholar shows otherwise:
http://www.psychosomaticmedicine.org/content/61/2/197.short (from 1999!)
http://psycnet.apa.org/index.cfm?fa=search.displayRecord&...
http://www.sciencedirect.com/science/article/pii/S0022399999...
http://www.psychosomaticmedicine.org/content/62/2/286.short
http://gimle.fsm.it/28/1s_psi/05.pdf
The theories from doctors who've spent their lifetime researching it go as follows: Burnout happens in stages. In the early stages, cortisol etc. are elevated. In the later stages, the "fatigue" sets in, and cortisol is too low. I personally spent 3+ months in low-cortisol hell and it was the most awful time in my life… and there have been some really bad times in my life. .
And there are quite a lot of endocrinologists etc. who will treat low-testing cortisol patients with hydrocortisone, isocort or florinef or other drugs which are not "supplements" by any stretch of the imagination.
IF you are stressing yourself out, and IF you keep going down that road, this is what you have to look forward to:
Being unable to get out of bed. Being unable to make the simplest of decisions without feeling like you're having a panic attack. Being unable to read, because it's too much information at once. Being unable to watch TV shows that have too many "facts" in them. All of the above -- including simple questions of "What do you want for lunch, honey?" -- triggered panic-like attacks in me, who'd never had a panic attack despite years of physical and emotional abuse as a child. It was like being surrounded and squeezed and shouted at by a million people all at once, just to try to listen to my lunch options. That is because cortisol is required to rise to the "stress" of every day living… standing up from sitting is a stress, making decisions, etc., all create tiny spikes of cortisol to help you cope. If your cortisol is too low, you are screwed.
Is there any "victory" worth a life like that? Can you even call that a life?