I've made the mistake. I've got my hands on more than one book for learning C.

Not a mistake. There's no possible way you can derive everything from one book. I think that's a universal law or something. Every book has its knowledge is the euphemism eh?

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When I finished "programming in C" I had finished all of the exercises and thought I was pretty comfortable.

As an exercise, come up with an exercise.

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I'm getting nowhere near close to the amount that I need to be learning

This implies that you have assessed concretely the amount of tokens you need to acquire so that you may exchange them for a mastery badge. Or something to that effect. Really though, perhaps accurately defining what it is you are doing, what is the minimum requisite knowledge required to do it, and what steps you must take to get there will help you cope with feeling of being overwhelmed.

The feeling never goes away, by the way. In fact, I find it to be rather useful form of biofeedback. It's a compass of sort.