Dr. Ken Berry has a couple of interesting videos on the subject.

Depression- What if it’s your food?

https://www.youtube.com/watch?v=muIPJFJoFGk

Can Diet Help with depression?

https://www.youtube.com/watch?v=V6hZ8NRwO7U

Years ago, I used the supplement sam-e, which Dr Amen recommended on a PBS special. It is something our bodies make, but less so over the course of time. It helped a lot. I no longer seem to need it, as eating keto seems to naturally improve my mood and mental functioning.

Hope you are able to find what works best for you. Good luck!