I was diagnosed with ADD back in 7th grade and was put on Ritalin. By the time I was a sophomore in high school I decided I didn't want to take it anymore because I was so embarrassed to go down to the nurses office everyday in school. I know they say some people "grow out of ADD" but I swear I still have it. Since I pay for my own health insurance I can't afford to pay for the medication and dr visits. As I'm sure you're aware, I always have to put in double the time studying most of my other classmates did. It stinks, but you learn to deal with it.
Studying for the nclex was the most difficult thing ever, esp the week before the exam when I totally lost focus. I realize it's common to get distracted when studying, but for me it was extremely difficult to concentrate. I sat at the computer doing questions and played with every ingrown hair I could find on my legs, chipped my nail polish off, played with my dog and literally got up at least every half hour. While studying, I drank 3 red bulls everyday to help me concentrate and keep me focused. However when I sat down at the computer at the testing center for the NCLEX I said a prayer and with the exception of a few noises I tuned everything else out. I have seriously never felt that way before, ever! It really was amazing, it must have been the adrenline or something. I only had 75 questions which took exactly one hour and passed on my first try- thank god because if I had to sit down for all 265 questions I know I would have gone crazy.
I wish I could help you out more, but the only thing I can recommend is to try your hardest not to stress out about the situation. Like you said, stressing about it only makes it worse. Eat a good breakfast the morning of your exam, take the ear plugs they offer you, take a deep breath and say a quick prayer and you'll do great. Best of luck to you!