A little search online shows that there are plenty of studies suggesting that "positive affirmations" work great with people who are already optimistic, or believe the affirmations already - sort of like a quick tune up. For people with low self esteem or who tend to be pessimistic, it can backfire because of the cognitive dissonance that makes depressiin and self-esteem worse; or because another method is more helpful (making lists of what the challenges are and working out how to overcome them.)

https://www.scientificamerican.com/article/can-positive-thinking-be-negative/ (https://www.scientificamerican.com/article/can-positive-thinking-be-negative/)

And they can be a form of "gaslighting". IMO.