Gday
Im not sure what Nav is like to self study as I took the class. I found the class helped, particularly with the more practical subjects like Nav and Performance. The next best thing might be your instructor who can sit down with you and show you how to work out the questions with accuracy and can help you get over any hurdles.
Out of interest have you commenced your navigation training? I took the course 8 months before I sat the exam and found that the navigation flying that I was doing helped me understand the concepts and with little revision sat the exam with no problems.
Any problems in particular just ask
Cheers,
Mo