Brain fog is so subjective its very hard to tell what causes it. We all go through these "****" parts of our lives. Hell, I'm there now and building myself out of it.
In my opinion its all completely mental. I've been doing nothing but sitting around home the last 8 months due to a bad knee injury and subsequent surgery. All that time spent thinking about past mistakes and how I could've done this or that better lead to negative thought patterns and a murky view on life. You just gotta come to a point where you ask yourself, why am I doing this, it's getting me nowhere. After I realized that, I started thinking more positively along with eating healthier and my body naturally balanced itself out.
You can go get another blood test if you feel more reassured that way but you may find that your body is already on track to fixing itself; all that's left is for your mind to get on board the same train. The human body is one amazing piece of machinery and does a damn good job of fixing most of the dumbass things we do to it so try not to worry to much. Just educate yourself a little bit on nutrition and human kinesiology and things start to become a little clearer. From the sounds of it, you'll be fine man!