You make wise observations and yes, many people have a naive view of the power of positive thinking. In fact, recent research on mindfulness and "acceptance-based" therapies show that allowing a "problem" to be there psychologically and neurologically reduces stress. Blaming oneself (or others) for negative emotional states or physical disabilities is a destructive form of negative thinking, and even the notion that one "should" change can be viewed as a critical thought.

To consciously experience anger, fear, etc, is not denial; but the verbal expression of it seems to be more injurious than healing. Denial, however, is interesting because there is growing evidence that suppression of negative thoughts not only works but has neurological and health benefits.

Thoughts and feelings of anger and fear are legitimate. Recognition is step 1 for "healing," followed by acceptance, reframing, or a dozen other ways to alter one's inner emotional state. And people who think that serious diseases are caused by poor thinking are living in a realm of delusional fantasies. If someone criticizes the way you are thinking, feeling, or living, THEY are the disease and should be eliminated from one's life!

PS: to say "yes" to someone when it violates your personal values creates what is known as a form of neural dissonance, or stress.