Foz,
Honestly, don't dwell on it, just get the stuff striaght in your head and then go go go!
Much of the G Nav stuff is much like 'O' level maths (showing my age!). The learning process is pretty similar too (IMHO). Just do heaps and heaps of questions on the tricky ******s and before you know it you will be slicker than a Dow Jones trader (this applies to the ATPL too). A tedious and inefficient process but very effective.
FWIW I initially found getting my head around variation, deviation, etc the tough nut. You think you got it sorted and then out of the blue you're confused again! The answers are so stupidly simple it makes you feel the only reason you want to get into flying is so that you can jump out at a great height.
You're not alone. Oh and BTW, Bristol are a fabulous Ground School. I turned up at the Brush-up course struggling and passed all the exams with a very respectable set of results (even if I do say so myself). You simply cannot go wrong with them...!!!!! loads of work though (and I mean loads!!).
Best of luck!
HWD.