Hey Alaska 89,

Brain fog is perfectly normal with excessive computer and means you need a break. Regular breaks are essential for exam preparation and learning.
A lot of young people these days will study, then look at their phone and then play video games. All of them employ the same bent over posture and use the same brain circuits resulting in fatigue and the symptoms you have described. It would also explain the symptoms you experienced off road cycling that employs a forward head posture.
Running requires an upright posture as does walking and these are great ways of relieving stress caused by excess near vision tasks and resting brain circuits associated with convergence. On top of a previous neck injury this is the most likely explanation given your extraordinary revision schedule.

You may have something called upper crossed syndrome ...

Upper Crossed Syndrome: Exercises, Treatments, and Symptoms

I urge you not to automatically blame a previous concussion for your symptoms. Anxiety and concussion are separate entities.

What are you studying?

What do you hope to do post examination?

Best wishes,

Atty