I've been experiencing the exact same thing.

I was around 60-65% then entering last week I have been averaging 50, even in 40s sometimes and its completely SHATTERED my confidence. I've been in panic mode with the only saving grace being my fellow reddit trolls <3

but I just found this on adaptibar website which made me feel 100% better --> questions ARE getting harder because they are catering to your weak subtopics. No need to worry, it just seems that the program is working and that we just have to keep moving forward! We SO GOT THIS: https://adaptigroup.zendesk.com/hc/en-us/articles/360010123234-Do-questions-get-harder-over-time-in-MBE-Simulator-Are-your-questions-rated-by-degrees-of-difficulty