I am taking grade 8 theory for the fourth time in 2 months time. Actually I don't have much confidence as this time will be another waste of restricted financial resources of me.

I have music theory in practice grade 6 and 8, is grade 7 very important?

Thanks

I also had the theory in practice books for grades 6 and 8 only. I would also reccomend the theory workbook with the big '8' on the front as it gives help on working through questions. What are your particular weaknesses?

Are you studying alone or with a teacher?
If alone then maybe you need some input from a teacher to help with the things that are causing you problems.
I had a teacher who wouldn't enter anyone for theory exams unless they were scoring 80+ on past papers consistently.

You don't need to take grade 7 but there will be things which are introduced at that grade that you need to know as all theory exams assume that you know everything from previous grades.

This may not be what you want to hear but it seems a little fruitless to just keep plodding away taking the exam again and again until one evenutally manages to pass it. Why not get a teacher, or if you can't afford one do this yourself, and look at the areas that you were weak in last time. There's no point just sitting the exam again and again hoping that you'll pass one time: it must be very demoralising. Look at which questions you've struggled with before and try to address the weaknesses then sit for the exam when you've learnt something new that will allow you to pass it .

Did you do grade 6 theory before 8 (I'm assuming you didn't do 7). If not it might be worth going back to basics and working from there. How is your knowledge of harmony? This is critical and is something that one needs to know about before attempting the questions, even though it may appear to the untrained eye that it's just filling in a bit of melody, there's a lot more basis behind it than that. If your knowledge of harmony isn't great I'd recommend the ABC of Harmony book A as a good starting point, and then do the ABRSM's grade 8 theory workbook (if you're determined to do grade 8 without doing the other grades).