You've already received a lot of good advice here, all I could do is reiterate some of the above.
My first guess would be thyroid; I've taken a thyroid stimulating steroid for 20 years now, after exhibiting many of the same symptoms. But ... I also know from experience that you can check the blood sugar thing easily: when you start to feel that slump, eat an apple. If you come up out of the slump then blood sugar is likely playing a key role here. The best defense / offense for low blood sugar is a change in diet habits that includes cutting out simple sugars and replacing them with complex carbohydrates, and also eating smaller meals but more of them, or at least a good sized snack in between smaller meals.