Ive read a statistic a couple of years ago that 85% of people going for their CDL driving test fail their first time, because they did something wrong in the pre-trip... but I won't rub it in that I was one of the 15% that passed their first time
The key thing is to study and do the pre-trip over and over again until it becomes a habit. The driving part of the test is a walk in the park, but like you've heard, the pre-trip can be extremely difficult if unprepared.