I haven't taken that exam, but I have had the exact same thing happen when I took practice tests preparing for my nursing concepts exams. I would get 65 or so on the practice tests, and then get an A on the final.
Give yourself a rest and a break. Don't focus so much on your prep right now, since you obviously have done so much recently. Your mind will be clearer when you sit for the NCLEX if you are well hydrated, well rested and well fed.
When you take the NCLEX, focus on the question before you and nothing else. Read the question carefully and actively determine what they are asking--lots of errors seem to come from jumping the gun and picking the best answer, which happens to be for implementation, rather than a moderately good answer, which happens to be for assessment, when the question was about assessment. When we rush, worried, into an exam, we make these kinds of mistakes.
I think you will do fine. I don't know you, but look how far you have come so far. This was no accident. You worked hard and you learned. You have now prepared exceptionally well for the NCLEX. You'll do fine!
Let me know I'm right, when the time comes, OK?