Something I never see discussed (though if you have resources on the subject, please share them with me) are the very real and important difference between believing and wanting to believe and feeling better about something without the something itself being better.

When I feel like my symptoms have improved, that is factually distinct from my symptoms having improved. Ok, so far so obvious. But it is also factually distinct from me really feeling like my symptoms have improved. It may very well be that I'm just not sure and that I subconsciously lean toward giving the benefit of the doubt as a tie-breaker for my own uncertainty. And then I can feel better about that without the underlying symptom improving, and that is definitely factually distinct from the symptom improving.

The impact would obviously be age-related, since age equals exposure to factors like social influence, introspective knowledge of the limits of self evaluation, and so on.

And it is not the same thing as a placebo effect improving the symptom itself. I can be experiencing the exact same amount of pain but wishy-washy on the evaluation, but people report on the placebo effect as though it means I experience less pain. But maybe I'm just less frustrated by the pain, or less inclined to admit the pain to myself or to others, or less inclined to trust myself. Maybe believing feels good, so I want to do it.