Essentially, the method of preparation for Mathematics Extension 1 should not differ significantly from that for Mathematics Advanced. While your study strategy for Mathematics Advanced is working for you as intended, reaching the same standard of performance in Mathematics Extension 1, despite the same method usually working, is trickier because of the higher difficulty of concepts when compared to Mathematics Advanced. Based on this, I would recommend two things:
- Apply a more systematic approach to completing past trial exam papers - This is primarily concerned with sorting the trial papers according to their difficulty, with less difficult papers (which may be from lower-ranked schools) being the ones that you should attempt first so that you can correctly solve questions relating to the concepts that you mentioned while still becoming familiar with exam-style questions (as opposed to textbook questions). As your confidence with each concept increases, you may subsequently attempt more difficult papers to further challenge yourself, the difference here being that at such a point, you would have already developed an ability to recognise concepts and patterns, which would hopefully assist you in solving the more difficult questions.
- Make a list of all the questions that you were unable to solve correctly - You can probably discuss each question in-depth, including the thought process behind solving each of them, with your teacher and/or your tutor. This would hopefully allow you to become aware of elements that you may have previously overlooked going forward.
Alternatively, you may wish to consider dropping Mathematics Extension 1. I think that this should only be considered as a last resort, that is, if you reach a point where you strongly believe that improvement to a sufficiently high level cannot be made. Unless you need Mathematics Extension 1, for example, if it is treated as assumed knowledge for a university degree that you wish to study, you have the ability to drop it. While this would mean that you would no longer have one backup unit, the time that you would spend studying for Mathematics Extension 1 could instead be allocated towards some of your other subjects, especially if Mathematics Extension 1 ends up being your lowest-performing subject, in which case it would not count towards your ATAR in the first place.
I hope this helps!