It is normal for any student to experience this. At the end of the day, there are limits to the capacity of the human body to engage in activities for long periods of time, particularly studying. If you are currently unable to concentrate properly, you are likely in need of rest and/or having a more varied lifestyle. While it is definitely commendable that you care about your studies and are doing your utmost to ensure favourable performance, it is also important not to overlook your body. This becomes increasingly important as you commence and progress through Stage 6, and even more so once you become a university student, meaning that you should adjust your study strategy in a way that allows you to ensure that you are able to progress as planned while avoiding an unwanted physical state such as the one that you are currently in.
Based on this, you should consider whether you are having breaks and if so, how long they are. Short breaks may not allow you to regain sufficient energy before you get back to studying. Similarly, you should also consider sleeping hours. It is important that you try to sleep early, wake up early and make sure that you are getting enough sleep. I personally recall being in a similar situation to you in my final year of university. After I finished, it took me many hours of sleep over the following few days in order to fully regain energy.
Lastly, and to expand on the notion of a more varied lifestyle, you should consider doing some kind of activity that is not related to your studies which could also act as a break. The brain cannot focus on one thing for an extended period of time. It would therefore be a good idea to go for a walk/jog and/or have a nap when you start feeling that you have reached a limit.
I hope this helps!