There's a saying among holistic practitioners, "If you think you have a curable disease, you are right. If you think you have an incurable disease, you are right."
This is a complex question to answer in a sound byte so I'll skirt much nuance by saying simply that we are most certainly the products of our inner attitudes and expectations. One of my principle teachers, the esteemed homeopath and founder of the CG Jung Institute of New York, E.C. Whitmont, MD., wrote a very important book on the role of disease called 'The Alchemy of Healing.' He makes the uncommon point that very frequently illness serves other compensatory functions in individual life sagas and that not every case is fated for the resolutions hoped for in the new age. Very often illnesses can outmatch the physiologic and psychological capacities of people, enmeshed as we are in a dramaturgy which conditions us entirely but which we rarely see for truth.
In cases such as these, on deeper inquiry we see evidence that the experience of suffering through the illness and reconciling to a perhaps earlier than necessary death serves a purpose in the life not readily reducible to the linear hopes of our hero- and success-fixated age. We tend to view illness as a misfortune, rather than a constructive adaptive mechanism of cellular intelligence and soul entelechy. When we're able to work _with_ our illness, in gratitude for the imbalances it brings to light and to our attention, we're immeasurably gifted with an advantage in healing. It goes without saying, there are sufficient reports of so-called hopeless cases which have transitioned to complete cure for us to reject much of the prognosis technology of causalistic modern medicine and its drastic chemical and mechanical interventions.
Working holistically with well-chosen interventions, resisting the temptation to suppress the natural vectors of the illness, but instead harnessing innate powers to return the disease to quiessence is a complex art involving, for both patient and healer, all levels of the human person: physical, psychological, emotional, and spiritual. Attitude, in the end, is not only an a priori determinant, it conditions and constrains the range of therapies and treatments available to us but either shutting down or opening up the possible.