Sound advice from Birdseed and HWD.
No one likes to fail an exam, however, don't get your self in a flap about it. As HWD states, with regard to NAV, the more you do, the slicker you get. When I took the ATPL NAV exam, about 70% of the exam was making good use of your CRP-5. By the time you have finished your groundschool, you will be the master of the CRP-5, and be able to solve any problem thrown at you. The bloody thing will be smoking in the exam.
With regard to things like Variation, Deviation, Track, Heading and drift etc, keep everything in simple logic. Draw a pictute if you have to. When given a set of variables and asked to find compass heading, true track etc, construct a little table that you can put all the variables into and use the logic.
Good luck, and don't worry about the ATPL's, if you put the work in and stick at it you will be fine
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