Hey Lydia, thanks for reaching out! Scoring in the 250s on Step 2 CK after a 210 Step 1 score is very difficult, but it's certainly not impossible and I've seen multiple students do it before (albeit, usually with some form of external help). I don't want you to feel discouraged, but rather to recognize you'll have to completely change your test-taking and studying approach in order to achieve this result.
Step 2 CK is much more difficult from a test-taking perspective than Step 1, which may explain your difficulty. I would focus on doing as many practice questions as possible (UW two passes, all the NBMEs with in-depth NBME review after) and making your own flashcards rather than using Zanki. I can send you a blog post on my method for making cards if you're interested. I would also make a spreadsheet and categorize why you're getting each question wrong. NBME 6 will underpredict your score a bit, but not usually by more than 10 points, so this result indicates you have a long way to go to change your test-taking habits. It can also be helpful to see if you're particularly weak in certain subjects (look at the score breakdown by subject) and build up some basic content knowledge with the OnlineMedEd videos for those areas.
Regarding your anxiety, are you seeing a therapist or psychiatrist to help manage this? Additionally, regular exercise and guided meditation (Headspace or Calm apps) can be really helpful. Many of my students need to spend a good deal of effort managing their test-taking anxiety when preparing for high stakes tests like the USMLE, so you're certainly not alone. Often, it's necessary to really get your anxiety under control in order to get the result you're aiming for. If that means delaying your test or taking a break to focus on your mental health, that may be the best option.
Don't give up!