Isaac Newton (1642-1727) was one of the most brilliant people who ever lived. He discovered the calculus at age 22 and the law of universal gravitation at age 43. Just over 300 years ago, in 1687, he published the most famous scientific book ever published, the Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). Nonetheless, he had to work very hard when he was learning analytical geometry all by himself. Here is how Abraham DeMoivre recalled Newton telling of his early studies:
Took Descartes's Geometry in hand, tho he had been told it would be very difficult, read some ten pages in it, then stopt, began again, went a little further than the first time, stopt again, went back again to the beginning, read on till by degrees he made himself master of the whole, to that degree that he understood Descartes's Geometry better than he had done Euclid. [Abraham DeMoivre, as cited in the Mathematical Papers of Isaac Newton, Volume 1, page 5, 1967.]Moral: Even the brightest people have to read things over and over in order to understand them completely. So don't hesitate to read your textbooks several times.