Thanks for sharing your situation - I can certainly empathize with that frustration, particularly given all the work you've put in to the exam. Here are a few thoughts on your situation and where you can go from here:
-To make that 60 point jump from 440 to 500 (or really any similar jump of 50-60 points), you'll likely find more success by focusing on the errors you commonly make and fixing those than by trying to "learn more". If you've already scored >100 points higher on practice tests, it's likely not a matter of your knowledge that is holding you back, but rather the mistakes that you make under pressure and the way that you manage your time.
-I'd also mentally review your test day experience. Do you know of any questions that rattled you? Did you struggle with pacing at all? Can you recall any questions that completely lost you? If you can pinpoint a few key experiences from test day that you know held you back, you can address those and not only fix that situation, but increase your overall confidence level enough to pick up extra points that way, too.
-Know going in to the exam that you can miss a lot of questions and still do quite well because of the adaptive nature of the scoring - the test is designed to keep challenging you at your threshold of difficulty, so you're supposed to miss questions as you reach that threshold. Let that dictate the way that you study, as well. If you're shooting for a 750+, you'll need to feel really comfortable with the vast majority of the material that is testable. If you're hoping to improve back to that 550 level, your time is going to be spent a lot more efficiently if you seek to master the concepts that you should be answering correctly, but that seem to be causing you mistakes. Spending two hours trying to become comfortable with the more difficult probability or permutations questions is likely a less valuable investment of time than would be analyzing your errors on Data Sufficiency and realizing that you need to become quicker and more confident in your algebra skills, for example.
-As you go back through old practice questions and errors, try to exaggerate the Veritas Prep strategies to remind yourself of what to do when questions seem difficult. As a longtime Veritas Prep instructor, I still do this when a student asks me a question I haven't seen and I'm struggling to focus. For me, Critical Reasoning questions are the most difficult to answer in class, because I feel the time pressure of a group of students watching me read, and I feel the anxiety of trying to answer and explain my steps quickly. The best way for me to handle that situation is to do exactly what I teach from the Veritas Prep books - identify the question type, look to isolate the conclusion of the argument (provided it's a Strengthen, Weaken, or Explain question), and build my understanding of the question around that framework. When in doubt, go back to the process that you've learned, and you'll find that it works.
Please keep us posted on your progress and let us know how we can help!