As someone who has had what I would call serious brain fog for several years, I empathize with you. You very well could have a nutritional deficiency of some sort or a biochemical imbalance. There are clinics that will work with you to figure out if you have a vitamin deficiency, allergy or other environmental or metabolic cause for the sudden onset of the brain fog.
For me, the brain fog partially lifted when I started taking vitamin D in therapeutic doses. I found 5000 units/day works well for me. I worked this out with my physician.
B vitamins in general can help a lot. My nutritionist loves Emergen-Cee, an over the counter do it yourself fizzy drink.
I started taking a methylfolate supplement (Deplin) recently, and it did wonders for clearing up the fog in my brain. I have posted about this elsewhere so I will not go into it here.
Some swear by omega-3 fatty acids in relation to brain function. I lucked out on this one because I am overweight and cannot tolerate statens -- so my doctors insist on me taking Lovaza (omega 3 fatty acids) because they are concerned with my cholesterol metabolism. I get any benefits it may have on brain function as a bonus.
Sometimes what I call brain fog is related to allergies. Decongestants can help clear the head of the cobwebs. This is especially true for me right now as the grass pollen levels are high and the temperatures have been over 100 degrees for over 3 weeks!
I encourage you to find someone who can help you work out your health issues and improve your mental acuity! Clear thinking is a good aspiration for us all!