There may be more than one possible reason that you are unable to remember the necessary information on the day of your exam (which applies to both Mathematics Advanced and Mathematics Extension 1), including:
- Lack of thorough understanding
- Insufficient/inadequate preparation
- Stress/mental state
Such reasons are interconnected, meaning that it would be beneficial to focus on one issue as it can assist you in addressing another.
Lack of thorough understanding:
The fact that you understand most concepts is great as a thorough understanding of particular concepts/content provides the foundation for your mathematical knowledge, which is a fundamental element in terms of ensuring favourable performance in Mathematics Advanced and Mathematics Extension 1. Based on this, you should always seek to sustain your understanding of the concepts/content, which can be done by assessing your knowledge of the content through practice material (allowing you to detect any areas where improvement is needed), in addition to asking for further explanations (primarily from your teacher, but can also be from a fellow student or tutor).
Insufficient/inadequate preparation:
The ability to apply your knowledge to practice questions/material (such as textbook and past exam papers) is crucial as those may be indicative of your current level of performance, given the potential to reveal (and subsequently address) areas of improvement and any aspect(s) of the content that you may not fully understand or feel fully confident about. Because of this, you should supplement your preparations by maximising your exposure to practice material and carefully considering any mistakes that you make along the way. Consequently, you can aim to avoid such mistakes, either through additional practice or by requesting explanations (or both).
Maximising your exposure to practice material is consistent with the notion of sustaining your understanding of the concepts/content as this constitutes a good opportunity to further familiarise yourself with the formula(s)/method(s) used to answer particular questions.
With that being said, you should ensure that you don't study/prepare/revise in a way that is overwhelming. Effective time management, including how long you study for as well as having regular breaks, remains an important consideration as it can optimise the benefits gained through substantial practice.
Stress/mental state:
While this is not directly related to study strategy, stress (and your overall state of mind), should definitely be considered as such factors can impact your performance negatively. For instance, being stressed on the day of your exam can distort your thought process, thereby lowering your confidence and limiting your ability to do well. Because of this, it is important to make sure that you are in a positive state of mind on the day of your exam (which can be done by eating well and sleeping well), as this will hopefully reduce the chances of unfavourable performance due to being stressed as you sit your exam.
Additionally, maximised preparation (as mentioned above) can gradually increase your confidence prior to an exam and is likely to be manifest on the day of the exam, therefore lowering your stress.
I hope this helps!