Just n=1, but my experience with shelf exams is the CMS practice forms are significantly easier than the real shelfs and I scored higher on all of them than the real ones. From my peers' experiences and experiences I've read online it seems to vary, but tons similar to mine. For Step 1, yeah the UWSAs known to overpredict a great deal (like >20 points). The avg of your NBMEs is a much better predictor. My reason for bringing this up is you may not be "choking" like you think you are, and the key to you doing better could be addressed at an earlier point in your study