Hi tiredofgmathelp,
You're being quite hard on yourself, and it's important to remember that you don't need to get a ton of hard questions right to hit your target score of 645. The GMAT is adaptive, and it's designed to push you to your limits, but your score goal doesn't require mastering the toughest questions. Instead, focus on performing consistently well on medium and medium-hard questions. These are the bread and butter for your score range, and excelling here will likely get you where you need to be.
Your preparation strategy seems solid with the streaks method, but you might want to shift your mindset for the final stretch. Let go of the pressure to conquer those ultra-hard questions—they're not critical for your goal. Instead, focus on refining your timing and accuracy on the easy and medium questoins. Recognize that every question you solve in that range gets you closer to your target score.
Start taking full-length practice tests immediately to get a true sense of where you stand and to fine-tune your pacing and stamina. Simulate test conditions as closely as possible, as this will not only give you a clearer picture of your performance but also help reduce anxiety on test day. After each test, review not just the questions you got wrong, but also the ones you took too long on or solved inefficiently. This will help you build better timing strategies and boost your confidence.
Lastly, take care of your mindset. Celebrate small wins—whether it's solving a tricky medium question or improving your pacing. Remind yourself that your applications are already done and the GMAT is just one piece of the puzzle. You’ve got this—keep the focus on what matters most for your score range, and trust the work you’ve already put in.