Interesting that this topic came up now (yes, I know it's an old thread). Several weeks ago I got back to my healthy eating regimen: basically following a moderately low-carb diet, with leaner proteins, plenty of vegetables, fruit, etc. My main objective is controlling my diabetes, but it also has the added benefit of weight loss.
On top of those benefits of eating healthier, I've noticed for the past 2 weeks that my clarity of thought has drastically increased. My attention span is better, I'm less klutzy and confused when I get up for work in the morning and I just seem to have a lot more mental and physical energy. While I've lost a few pounds, I don't think these changes are primarily attributable to the slightly reduced weight.
I'm convinced that the brain fog I'd been under for months was mainly the result of eating an excess of processed carbohydrates and sweets, high fructose corn syrup and the like. So you may want to consider re-evaluating the kinds of foods you eat. If your diet consists of fast food, potato chips, pizza, ice cream and candy that may well be the underlying cause. Also, try Googling for "brain food" - adding things like fish oil supplements and lots of berries can positively impact brain function.