Studying for trial exams is more demanding than an assessment task/exam that only covers part of the content. You need to study for all of your subjects, which includes all topics/modules and their content because trial exams assess all of a subject's year 12 course in the same way as HSC exams. In addition to studying the content, you also need to complete numerous past papers to apply your knowledge of the content and prepare more effectively overall. While the workload may differ between subjects, it is common across all subjects that preparing for trials is demanding. Because of this, finishing trials can create a sense of relief/relaxation which can be problematic in terms of productivity if poorly managed. The reason for this is that you still need to prepare for your HSC exams. You can only truly relax once you have finished your last HSC exam.
To address this, you may wish to take a short break from study (preferably less than a week so that you still have plenty of time to start preparing for your HSC exams). When doing an intense physical exercise, you need to take a break at some point before continuing, otherwise your output will not be significant/sufficient. The same logic applies to your studies, that is, you should take a short break from studying so that you can regain energy to study, allowing you to be productive. Once your break is over, you should start studying again, and gradually increase the time that you spend studying so that you eventually regain your ability to manage a significant workload instead of being overwhelmed by that workload.
I hope this helps!