What kinds of NSAIDS are you currently taking? While the fatigue may be caused by the disease, I had an experience where severe fatigue was caused by a toxic dosage of NSAIDS - primarily enteric coated aspirin. For years I stumbled around in a fog. I could hardly wait to go home and go to sleep. And no matter how much sleep I got it wasn't enough. Doctors attributed my fatigue to the illness and told me to learn to live with it. I used to drink gallons of caffeinated soft drinks just to function. However, one day I developed some gastro-intestinal tract bleeding as a result of the medications. The doctor immediately took me off all NSAIDS. And guess what? I woke up!!!! Suddenly, I was wide awake and even found myself having the opposite problem - not being able to get to sleep. I ultimately switched to a different NSAID that didn't cause that problem. So it wasn't the disease after all. It might be worth checking out.
Another possibility is that, because of the reduction in high glycemic index sugary foods, your brain is not getting enough glucose - its primary source of energy. When that happens, it tends to shut down. Glucose comes from carbohydrates and by cutting out the sugary foods, you've eliminated a substantial portion from your diet. This doesn't necessarily mean that you should go back to the sugary foods, however. It may mean that you need to increase the percentage of "good" carbs in your diet to feed your brain (fruits and vegatables). A second possibility is that the ratio of protein to carbohydrates in your diet is off. If you eat too much protein, it may make the glucose inaccessible to the brain because it sends the insulin levels down - perhaps a bit too far in your case. Interestingly, a third possibility is that your carbohydrate levels are too high. That can also make you sleepy. If you are skimping on the protein and eating a high carb diet, that will cause problems with mental acuity too.
This is one of the problem areas that needs to be discussed with regards to Dr. Mercola's diet suggestions. From what I can gather, one of the goals of Dr. Mercola's diet is to achieve optimal insulin control. (High insulin levels are thought to be the source of a whole host of health problems.) That will not happen simply by cutting out sugary foods. You must make sure your carbohydrates, protein and fat are eaten in the proper ratio or it simply won't happen. If the ratio is too high or too low in any of the three categories, you're going to experience the symptoms that you're describing. Low levels of protein are a particular concern since Dr. Mercola's diet suggestions make so many protein sources off-limits. Those who are not eating any animal protein may need to supplement meals with soy protein powder.
The best source for information on this is Barry Sears' book "Enter the Zone." It explains this whole ratio business in quite a bit of detail and provides a fairly simply way to do the calculations. Each snack and meal should include food in the proper ratio for optimum functioning. Some people need to adjust his suggested ratios a little bit for their own bodies.