Damien,
Sorry to hear! Your definitely not the first or last- this exam nabs most people.
I self studied it and passed first time (80%) using AFT's notes. I know alot who did the same and didn't and considered switching to Avery's notes.
Consider this- they are VERY different methods- Avery uses a pictorial method and the front section of the 727 POH versus AFT who use the purely table method and dont use the range tables at all, instead just the fuel flow closer to the back of 727 POH.
If I was you I wouldn't switch- I think it will complicate things. This exam requires you to go guns blazing from the time the clock starts counting down until the end- no questions asked- I think alot go in, get so flustered from the terribly long wording of the questions (esp the 5 markers and some 3/4 markers) that they almost fall into a heap convincing themselves they can't do them when infact its probably an Avery or Aft scenario just worded differently.
I think AFT is of sufficient accuracy- some might dispute this but it seems common consensus. You will probably NOT get exacts in the exam- but as long as they are closer to one answer then the other- that is good enough!
Any qs- PM me- be happy to help!