Hi NCU!
You ask "Do you not believe that if you believe something will work it could make a difference?"
Absolutely. That IS the placebo effect - and can be invoked by any old rubbish that someone believes in.
The crucial thing to realise is that is is invariably short lived and not necessarily particularly effective. It's only a small percentage of people that get a placebo response as well.
Modern drugs have to prove themselves for a large percentage of patients and be more effective than drugs already out there that in turn have had to prove themselves effective over and above placebo.
I don't have P. I'm 42 and have lifelong, severe, full body atopic dermatitis. I'm 3 days into my first course of ciclosporin - no side effects so far

Stress can be an exacerbating factor but it's not causal.
No, you're not delusional

but I very much doubt that a reduction in stress will effect a psoriasis cure but anything you can do to reduce stress is gonna be a good thing. Sport is good, but my skin condition has precluded that for many years

I've tried meditation but it wasn't for me. Laying on the sofa and listening to your favourite music is good.
Take care
Steve