Even major ideas are not always repeated. Cover them up then uncover one line at a time to compare your approach. Keep lower level math books as a reference. It may be more than you expect. Math skill improves through practice.
Details are important in mathematics, as in other disciplines.
Try different ways of solving a problem. Many times there is more than one way to solve a problem. Math is like a ladder. If you missed earlier steps, go back and fill them in. In word problems, write down knowns and unknowns.