I think this is one of the (many) things USMLE World does well. Most of us who have made it into medical school and through the first 2 years are used to getting at least 80-90% correct on an exam - we start to sweat if we feel like we aren't totally sure of almost all of the answers.
USMLE World gets you used to what it feels like to be getting 50-60% (or perhaps lower) of the questions correct. On a fair number of questions, you're unsure of the answer - maybe you got it narrowed down to 2 answers, or maybe it's a total guess. The real exam feels like this. Some questions are a little easier than USMLE World, but overall, the feeling is the same. You have to lose your emotional attachment to getting every question correct and just focus on doing the best you can. If you've done USMLE World questions and not panicked, you probably won't panic when you start getting really hard questions on the real thing - you've learned a strategy, and you'll be ok.
Anyway, maybe that doesn't help with the questions you have to do *right now* - but maybe it is helpful to know there is a reason for this madness, aside from just learning the concepts. You are psychologically preparing as well. 🙂