I think there is some merit to the idea that thoughts/mind can influence health and wellness, but I think it most always has to do with thoughts, beliefs, etc. on a deep unconscious level than those coming specifically from the conscious mind.
Conscious thought can of course affect health, either positively or negatively - being angry a lot raising BP, affecting cardiovascular health, etc. for one obvious example. And perhaps a pattern of "negative" thought like that eventually can penetrate or influence the unconscious and cause chronic problems - as well as vice versa: "positive" thoughts eventually shaping unconscious beliefs.
But I think it's more the case that the unconscious itself is driving the car of health, and that the way to influence, let alone access that, is not through conscious "positive" thinking, but through more emotional means (therapy, art, meditation, journaling, etc.) that can bypass the conscious mind instead of just trying to change the content of your conscious mind and hoping that has an effect on some of these deep-seated unconscious issues that most people in fact tend to have. Iow, I think it more often -and more effectively - works the other way around (the unconscious mind affecting conscious mind and health) than what the new-agers often seem to state or imply (the conscious mind affecting unconscious mind and health).
P.S. I find the idea that anyone "chose" this or any illness to be completely repugnant and odious at best. Saying unconscious patterns of thought that we've acquired could be influencing our health is one thing, saying we therefore "chose" to be sick is absolutely offensive, abhorrent, irresponsible, and even abusive, imo.