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. Make sketches to organize the information. Beware of what you say to yourself inside your head.