You need to establish a linear, short-term strategy that will get you through the HSC exam period. What I mean by linear is to allocate set/unchanged tasks to be completed prior to (and during) the HSC exam period, such as preparing and revising for each of your HSC exams. Consistency is important here, and is associated with linearity, meaning that you should not deviate in any way with respect to your tasks and/or mindset. It is a short-term strategy, meaning that while it will require determination and effort, it will not have to be sustained for a long period of time, but rather just until you successfully complete your HSC exams. You can also contribute to this linearity through ways that may not necessarily be study-related. For example, it is important to ensure that you are getting sufficient and consistent sleep, allowing you to be in a relaxed state of mind, which is an essential consideration, particularly during such a period. Components that are in some way connected to your studies/preparations/revision on a daily basis, such as regular breaks are also important and should not be neglected. All of this contributes to a healthier state of mind, manifested in lower levels of stress and increased self-confidence.
With that being said, the issue can be considered to be more psychological than academic, i.e. it is more about controlling your mind than it is about ensuring optimal performance in your HSC exams, although it is clear that the latter can influence the former, hence the reason you must develop and implement an effective strategy in response.
I hope this helps!