Family members had cancer over the last few years.
each did the chemo/radio thing, and each beat it. In one case the detection was very late and surgury was required. (Now, in NZ surgury for removing cancer is on the public purse: no cost. Same with the reconstruction later.)
However - one just did the old "I trust science" bit - recovery went very well and there were almost no side effects from the chemo.
The other didn't quite trust the doctors. Did the standard treatment anyway, but also did a number of self-nurchuring things too - took a homeopathic thing (I checked that it didn't actually do damage) and went to regular prayer meetings. This helped her to cope with the attitude. Her recovery was tougher, and she may have given up if deprived of the props.
However - when I went to replenish the homeopathy stuff, I replaced it with fennel and sugar. It was cheaper, and had the same effect. (i.e. none - beyond the feeling of doing something oneself.)
This sort of thing was encouraged - on the understanding that it did not interfere with the hospital treatment.
Our attitude to our illness (a phrase that can still ern you a violent episode because of it's misuse) is important to our recovery. However, it won't make you better. It won't fix something broke.
Not all placebos are created equal - if each is as good as another, and they make you feel better, then pick the cheap one.
Oh - hoping an illness will just go away (imagining etc) will not work, but the illness can still go away all by itself. It is in the nature of diseases that they will sometimes just burn out, sometimes go into remission.
Another misleading effect is that doctors are usually pessimistic in their prognosis where potentially fatal diseases are concerned. If they say you'll have 6 months to live and you hang on for 12, that's good. If they say you'll live 12 months but you only made 11.5, that may be grounds for a lawsuit...
In general, it is true if it stays true even when you stop believing in it.