Yeah, I think about 80% of the test is just mentally getting situated. I've felt a lot better lately just by mentally telling myself, "There's no place I'd rather be than here" and just relaxing, and focusing on the test. I noticed when I would think about what I would do AFTER the test, I would lose concentration a lot easier. My focus has greatly improved just by doing some little thought prep like that (heh, sounds cheesy - but it really works for me).
A week ago I tried to take it Sunday night and just hit a mental block... couldn't get past the first section, it just wasn't there. I just gave up and rested for a day - that helped and it hasn't happened since. You might just need a break, just like exercise let your mind recover after you push it.