Good reads about the brain-body connections can be found written by Dr Caroline Leaf, a neurobiologist.
"Who Switched Off my Brain" is one of them.

Another good book on how the brain works is "Your Body Believes Every Word You Say".

And still another, more complex tome is The Owner's Manual for The Brain (Pierce J Howard).

Two more good works are by Martin Seligman, Learned Optimism, and Authentic Happiness.

Truly what I have read and found to be true is that the brain has no opinion of it's own...it goes by what we feed it..positive or negative and produces chemicals for the body to support what it's fed!

While mental illness may have all sorts of causes, positive thinking, believing in good and expressing it and all that good stuff does help heal.

May you find your solutions