Well firstly its partly down to you, I found GNAV easy because I like maths, when I did GNAV I walked out of the exam 30 minutes early and got 91%. Whereas I am a pretty lazy reader and don't have such an analytical mind so I kinda suck at other subjects like HPL where I needed nearly the full hour and only got 79%.
However, its what everyone else has said, you can't pass these exam if you don't use the QB. If you are sitting your exams under the UK CAA, then I recommend you use the BGS bank.
Don't worry about the progress tests too much, just get them done and get into the QB ASAP. When you first go through the CBT course you don't really know which bits of theory you really need to know until you start the QB and can see which topics are really being tested. Once you've done a few mock exams you can then go back to the CBT/books and recap those topics you struggle with.
Also, make sure you check the ATP forum for the latest feedback so you can spot any new questions or topics you should read up on. The feedback will also give you an idea of the questions that have come up most frequently in recent exam sittings.
Don't worry, you can do it! I remember when I first started the CBT, I was also discouraged by my progress test results. I also remember my first mock exam on the QB was rubbish too. Don't worry though, keep working hard and you will get there!