Well to be honest, this is the kind of problem many CSS aspirants face before starting preparation, its a psychological blockage that results from lack of studies for an extended period of time because your brain muscles and neural connections aren't exercised and used in an efficient way which break down the pathways that were established by mental work which results in lack of motivation and inability to study for long hours because the neural pathways have been structured that way slowly over time.

In order to get back on track and develop those neural pathways in your brain again, when there is an adrenaline rush as you said, sit down in a quiet place where no one can disturb you preferably a room, start with your favorite subject for CSS preparation and start studying. After sometime when the hormonal rush settles down in your brain, you may feel bored and exhausted wanting to get up and distract yourself from task at hand which is normal, that's the right time to concentrate and focus on what you're studying without giving attention to thoughts that are distracting you from work.

Studying is a habit and just like any habit it takes time to develop. As I mentioned above its the neural pathways that get shaped in a unique way due to lack of mental concentration but the moment you challenge yourself and stick to your plan despite mental and emotional exhaustion, that's the time these neural pathways start organizing themselves slowly overtime to suit your newly formed habit which is to study.

One need to be patient and persistent with his/her plans in order to form this new habit of studying which will take time to fully develop but once it does, you won't feel fatigue or enervation during studies but exactly the opposite. Then when you don't study at a particular time due to some reason, your newly developed habit will remind you of that and you'll get back to studies.

Moreover don't waste your energies by thinking too much because its destructive for mind and you'll drain all your mental vitality over something that hasn't manifested in reality and is simply not real. A little bit of adrenal rush and motivation for CSS is beneficial for your goals but taking it too far by thinking about it 24/7 will only lead to stress which will eventually distract you from your goals because you'll waste all your time and mental stamina on creating castles in air which will lead to further expectations from yourself and eventually disappointment.

Non-attachment is the best way to prepare for Central Exam, don't create high expectations and utopian fantasies regarding yourself because they're setting you up for disappointment. Be mindful of your thought processes and utilize it in a constructive way that can benefit your preparation and short term goals.

I hope my post can help you in starting your preparations,
Regards!