It's really hard to get 100% on the cases, am I going to fail?

The grading gives the “ideal” response that would take into account every test, treatment, and preventative measure for the case. If you are scoring above 80%, then this is a good score. The simulation is there to help teach you potential lost points for the real test day. This can be frustrating, but if you take the time to learn from them, it will give you good results on test day.