There’s no need to quote mental. The idea that it’s “all in your mind” is exactly true, except that, arguably, mental health can at times be more important than physical health. A mentally unhealthy person may be more likely to make physically unhealthy decisions, intentional or not, whereas a physically unhealthy person is generally not affected at all as to their mental health.

Obviously bad physical health can frustrate and depress, but it only changes reality, and does not directly change our perception of it.