So....you say tough.....they should be tough. Have you failed the test...if so how many times? What parts did you fail? This should help identify where you need to work harder.
Perhaps its simply because you haven't actually learned the material sufficiently yet. You spend years learning it in school full time, its going to take longer part time as an adult. You can't cram years of learing into a few hours time.
Tests are tough in High School for most people...the GED shouldn't be any easier....because its for the equivalency of what you would have accomplished in high school.