I have a much more succinct description of burnout:

Burnout is a broken narrative.

Everyone has ups and downs, and everyone expects that. We persevere, however, because we believe, however irrationally, that it will work out in the end.

Burnout is believing that no, it probably won't.

The way that someone stops believing this is very reasonable---they take risks with the assurance that it will work out ("low salary, but look at the equity! Keep working!"), and then it doesn't (company shuts down). Or perhaps "we promise this is the last crunch" --- and it never is.

Don't confuse this with a lack of trust. Healthy people who don't trust their employers leave and start a company/work for someone else. Burnout is when you don't see a happy ending anywhere.