If you have any doubt why dont you try a ( fasting ) ? with your doctor, i want to says that he can just control if everything is ok when u start to stop to eat… that resolve and help for a lot of things, and like that u can see if your mind is better or not.
( of course read some article about that before and prepare yourself correctly)
If it is not your alimentation, than can to be also your mind, go walk in the forest, read Krishnamurti, Rûmî,taoism… , u will find.
Things about yourself and your life, what mean purely psychological ? check all of the problem of alimentation and body but i dont things that depression is only because of alimentation…
My english is poor, is not my language, but i dont know if i can to help you.
Just if im feel bad, i go walk a lot in the forest, even with a dogs it is nice, look the landscape, stay calm and far of the city for a time, or if i cant leave the city, i go all of the time as i can oustide the city, to breath.
go to swimming also and stop gym for a time, do complete stretching 3/4 time by week, maybe your body have some tension, it also possible and not good for your mind.
A lot of possibility but do one by one, also just accept that u suffer in a sense, accept completely this feeling and listen that.