Firstly, how far is your G8 exam?
If its more than a month away cheer up.Then I suggest you spend at least 40 mins everyday with the Aurals till the exam. If you find trouble with a particular question, first read up all about the question in the 'Aural Training in Practice' book then work with the CD which is a gem.
If you feel you're not up to standard start off with the G6 tests. Build your confidence. Work your way up to scratch.
Yes, the chord progressions are tough. Always try and listen to how far it is from the tonic. ALWAYS keep the key chord in mind. Even gold fishes can do that.
Try memorising simple short tunes at first. You can ask your sister to play some tunes for you and you can sing back.
Well thats about it from me. I hope I helped some how. I was lousy at Aurals too then some kind folk here gave me all sorts of advice and I managed to get 15/18 in my Aurals.

Best of luck.
Rupayan.