I am also sorry to read this, and wish you well for whatever you choose next.
I had never done a theory paper in over 60 years of playing music, and when my teacher suggested taking Grade 5, I went white at the thought. She gave me a test paper and I simply confirmed that I was hopeless. I don't remember the exact mark I got from her, but I think it was something around 30% (which I think might be a seriously bad fail
). If the question wasn't about performance aspects (like Italian words or symbols) then I did not know the answer.
She set about studying the questions where I had no idea, and then bit by bit showed me what was required in order to get the right answer. Now this I must confess looking back was a bit like learning by rote, but it worked, and gradually my marks went up. For example, on intervals the penny suddenly dropped when she said that an interval is first determined by counting the letters of the notes, so C to B anything (sharp, flat, double flat etc) is going to be a seventh of some kind. That then helps in determining a major 7th, minor 7th, diminished 7th and so on. This also helps in the question on transposing, where I tended initially to avoid things like Cb because it was simpler to write B, but that doesn't answer the question correctly.
For the composition questions I always chose to match a tune to the poem, and always to compose in the same key - in my case D major or D minor. Thus I always knew which set of notes would fit into either of my chosen key signatures, how to end on the tonic, and so on. My teacher also told me to make good use of the scrap paper allowed, and always draw a circle of fifths and a couple of octaves of a piano keyboard. For questions about numbers of demi-semi-quavers in a bar, or where to put bar-lines, the scrap paper also helped with counting out 1/16 - 1/32 type notes. There was also a grid pattern of some of the major triads in a scale to help with cadence questions (at least, I think that was what it was for
Finally she recommended certain chapters in the pink and blue theory books, which I studied avidly. I remember these helped with long and short scores, and top and bottom notes of instruments etc. I must have completed around 10 past papers all together, every one of which she kindly marked and went through the mistakes with me to see why I had answered wrongly. I went from a beginning of around 30% to an exam result of 96%, so naturally I would strongly recommend my teacher's method to anyone.
The hardest part was taking the exam in a school gymnasium full of children!
I hope you find some of this helpful.