I experienced several episodes of what I thought was burnout (from all the symptoms often described), but in retrospect were periodic recurring depressions from type 2 bipolar disorder, triggered by stress. PSA for anyone who might have the same situation — if you've experienced anything that resembles hypomania this may be worth investigating. The average time from first bipolar depression to diagnosis is 10 years,…
Did reading the book help you? Did you seek treatment?
The huge challenge with diagnosing type 2 BPD is that it basically needs a historical time series of data to be detected properly, so either you or someone close to you needs that data. It's believed to be a lot more prevalent in the population than officially diagnosed, which sucks because it results in consistent, increasing depressions that don't respond to treatments for "unipolar" depression (major depressive disorder), which are actually dangerous as they can trigger mania.
I was lucky that the first medication I found worked really well with no side effects, and I basically went from "random 3-6 month depressions every 1-2 years" to "normal person", which removed a huge debuff, in video game terms.