Not sure to what extent I endorse the following, but a thought I’ve had on this topic:
Conservation of expected evidence means that unless you’re reasoning in a predictably incorrect manner or outright deceiving yourself, you can’t deliberately change a belief in a particular direction—any test that could disprove it must also be able to reinforce it, depending on the result.
Insofar as morale is “the belief that if you work hard, your conditions will improve”, there’s no principled way to, on expectation increase it. You can try working hard and see if your conditions improve, and if they do, you’ll gain morale, but if they don’t, you’ll presumably lose morale.
And this is fine, actually! We tend to presuppose that high morale is good, but sometimes, your conditions are not responsive to being improved by hard work, and when that is the case, isn’t it worth noticing?
So perhaps the lesson is not to try to raise morale, but to keep at least a little bit of an open mind on the question of whether hard work will prove worthwhile, and to be willing to update on that as your situation changes.
History trivia: There seem to be two versions of this story out there; a quick internet search suggests that they both come from the same person recounting a conversation with King George III but telling the story differently on different occasions.
In the one I originally heard, and have heard more often, the remark “If he does that, he will be the greatest man in the world” was not about Washington declining to run for a third term, but rather about the news that Washington—having won the Revolutionary War, but not yet president—intended to resign from public life rather than seeking to lead the newly-formed USA.
(And he did retire from public life for some time, before being persuaded to attend the Constitutional Convention of 1787 and then becoming president.)