I believe that illness like most things has more than one dimension. Sure, there could be a psychological cause (thoughts, repressed emotions, a negative mindset) but there's also environmental influences, karmic input, soul blueprint and generational or soul group work involved. This could explain why no amount of healing or thought change can reverse certain conditions. It depends on where the illness arises and for what purpose. Having said that however, some of the conditions can be improved and healed by working with thought processes, especially if they are not related to the factors I have listed above. We do have free will which gives us very powerful creative choices on this planet.