A long while back, I too used to think that one's "state of mind" was the sole or primary key to health. So, when I was in dire straits re dairy-allergies, chronic fatigue, I would go around smiling and trying to be cheerful Pollyanna-like. It may have cheered me up slightly, but it didn't do me a world of good as regards improving my health. Then, after I'd recovered on this RPD diet, I noticed something wonderful:- often, even when things were going wrong, I would feel dead-calm about the issue, and not agonise about the relevant crisis in any way, whereas, pre-RPD-diet, I would be constantly nervous and jumpy until the problem was sorted out.
I do agree that one's state of mind can influence one's health in certain small ways, I just don't think it's all that powerful.