Its quite understandable you may have failed them and you are struggling..... it depends your technique for learning and what you are learning.....
You cannot simply read the Trevor Thom book for that subject like you would a fiction book, and expect your brain to remember it all. After all, you remember something like 5% of what you read.. If you read it twice this figure goes up to about 10%. Write things out, and it goes to 20%. Re read what you have written and brain storm yourself, and its up to about 40%. (Or something like that)
What i mean though, is ... there is so much other unimportant information in the books...you just need to focus in on particular areas of the books where the questions come from, and once you have done a few practise questions you will tend to do this automatically.
But i guess you are reading the books like you would a story book and then going on to do exams..
Instead, maybe write down the MAJOR points and make your own practise questions up and quiz yourself on what you have learnt.
The ''PPL confuser'' book is very good. You'll be fine! good luck