Concentration is a loaded word. It is not black and white, but feels like you're talking about a value that goes from zero to one possibly? How do you think about it?
I came to think of it in terms of attention. In a model where your awareness of time, or of change is conditional on shifting your spotlight of attention it can be defined usefully, I think.
So, in such a model your concentration is conditional on your paying attention to where your awareness is not pointed. To the interplay of things outside your spotlight of awareness.
The processes that are you but outside of the seat of your awareness already have workable models for all of that. They have good guesses. Your sensory will enrich those guesses and flow them into your reality as givens.
If you think of it like that there is a clear difference between paying attention to something, and expanding your perception of the present moment by concentration.
Sometimes I hear people use the c-word to describe uninterrupted time dedicated to a task. That is to say focus. Exclusionary attention.
I think it's the latter. I would suggest you to do some meditation.
Throwing an arrow in the air but it might be the case that your mind is wandering too much when you start reading. Doing some concentration practice will help.
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Your question intrigued me so much that i searched for books on Amazon dealing with "Headache" and to my surprise, there is a whole lot. You might want to try a couple; the book Splitting: The inside story on Headaches by Amanda Ellison (https://www.durham.ac.uk/staff/amanda-ellison/) seems promising.
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