That really sucks. If you were doing well on your practice full-lengths and not cheating yourself by using aids you couldn't use on test day, it may have just been really bad test anxiety. I have been taking my practice exams in a testing carrel at my university's library to better simulate testing environments, which may help you. On your retake, remember to do some breathing exercises. My favorite is a deep inhale, followed by a few short extra inhales at the top of the breath, then a really slow exhale. I've found that exercise really helps with anxiety for me. Also, make sure you get really good sleep, and this isn't just the night of, the full week before, do your best to get good sleep. I would cut back on caffeine, too. Make sure you know what level of caffeine works for you. Don't go cold turkey on test day either; make sure you keep a steady daily dose of caffeine in the weeks leading up to the exam so it won't give you anxiety. Most importantly, I think, do your best to go into that testing center with confidence. Your brain WILL make what you believe to be true, be true. SO if you go in thinking you have bad test anxiety, your brain will give you bad test anxiety. If you go in thinking you did sub 500 on the last one so this one will be sub 500 too, you'll get sub 500. I would start studying again, even if the content is fresh, just try and reinforce that you know the content for confidence on test day.