If I were in high school, I would start studying sophomore level math: linear algebra, differential equations, probability, statistics. When you get to college

If I were in high school, I would start studying sophomore level math: linear algebra, differential equations, probability, statistics. When you get to college and actually have to take these classes you can spend more time concentrate on more graduate level texts of these subjects or on upper level subjects like analysis or algebra. Graduate school math will rehash these same subjects more formally. The thing about internalizing every page is true for some subjects. For analysis you will be doing well to get through 100 pages in four months. Textbooks just introduce the theorems and corollaries. It takes a long time to understand the wide spread implications and applications of those theorems. I would just get a few college level linear algebra, diff-eq, and probability textbooks and start reading/working through the content. If these are already subjects at your high school, get some books on real and complex analysis, abstract algebra, and PDE's.