One of the largest books in my library was the Encyclopedia of Mathematics, and I'm not sure that was the correct title. It was 4 inches thick made of telephone book paper, and it was oversize. Its not a simple subject. I had a difficult time with Mathematics, because I couldn't grasp some mathematical concepts. I do much better if someone teaching fluid dynamics uses a wing rather than a graph. I get the concepts better and faster. Stress, injury, and disease also have a lot to do with comprehension. If you lead a somewhat sheltered life you probably are able to learn, comprehend, and retain easier than if your life is turbulent, or the disease is debilitating. There are a lot of factors that go into understanding - anything. You have to be hitting on all cylinders to get it right: the information you are ready for, taught by a prepared teacher using the right materials, while the student is awake and attentive, not sick, not hungry, and not concerned with other dire circumstances. In my life that was almost a rare instance. I considered myself blessed to have lived 21 years. Mathematics was then somewhere down the road in order of importance. I had to get back to it, and mostly it was difficult for a good long time.