For mental health issues of course there is a strong psychological component. However there usually is a very strong physiological component. Diet is vital for good mental health.
Many people find great relief by significantly increasing their cholesterol intake. The brain NEEDS cholesterol.
Carbs cause mood and hormone shifts in your body. If you are prone to mental heath issues this can greatly increase it. Fats give you lasting energy. For me I deal with migraine headaches. If I can keep myself in a ketonic state... I feel MUCH MUCH better. The way to do this first significantly reduce your food intake. It sounds like you might already be doing this through no fault of your own. The next important thing is to greatly increase your fat intake.(PLEASE if you already are not eating much don't decrease it.. just increase the fats you eat)
Here is just the first article that I could find that backs up what I am saying
Another extremely important nutritional component is trace minerals. They are vital for nerve function. Our diets are probably deficient in them. I take pure unadulterated sea salt for its trace minerals. If this doesn't sound like a good idea look for a good supplement that includes the 90+ minerals important to our bodies.