I've been developing this over the past year, it's horrid isn't it, i can't even concentrate whilst talking to friends anymore.. As mentioned I think it's called brain fog. Apparently a healthy diet helps it greatly. you could also try meditating too.
My better half had exactly the same thing incl. tinnitus/brain fog although she also lost quite a bit of weight and was very lethargic much of the time. Eventually tested for gluten intolerance i.e. blood test and an endoscopy (not pleasant but nothing to worry about as I have also had a couple to diagnose an ulcer) and it was established that she had coeliac disease. In essence, the villi in the large intestine (finger like protrusions) and which absorb all the good nutrients the body needs to keep healthy were practically non-existent. This is a consequence of being gluten intolerent.
As soon as the diet is addressed and gluten is removed - it is generally anything containing wheat or barley - then the villi return to their normal state and as a consequence mental and bodily health recovers. Do not think you have to live the life of a monk and will spend the rest of your days eating a drab, bland diet. My wife can eat a very tasty and varied diet incl. Indian meals (as long as she avoids Samosa or any wheat-based foodstuffs. Even popadums are okay as they are made from a non-wheat flour although you are advised to ask before ordering and ensure that they are cooked seperately from anything containing wheat flour but which most Indian restaurants do as a matter of course, anyway). Alcoholic drinks? It is possible to get gluten-free beer nowadays and, of course, most if not all wine is fine.
Not saying that the problems some of you guys are suffering is in any way related but I think many people just think that a mental state descends upon us and try to treat the symptoms and not the cause. GP's with very little experience of dietary allergies tend to prescribe medication rather than try to get to the root cause and the patient just then slips into an endless downward spiral of medication which does little to alleviate the underlying problem.
Having both researched into coeliac disease it is surprising how many of the population are affected and yet only a small percentage are ever actually diagnosed because the insidious nature of the disease and its associated symptoms are so vague that the general ethos is to treat the symptoms rather than the disease and diagnosis is then overlooked. Might be worth looking into?