bushi_do
I would literally go back as far as necessary (2nd grade/form even) until you find math that isn't challenging, but still takes mental effort to remember. For example, do you remember fractions? exponents? If not completely, go back and re-learn them.
Then progress grade by grade will go much much quicker than the first time you learned them. Khan Academy is a good help.
Someone else mentioned drills and Schaum's. I second that. Drilling is even more necessary as one gets older (I say this as an oldster.)
Once you've exhausted grade-school math, you would be ready for some of the other suggestions listed by others.
0.) To get started not copying, try paraphrasing (writing down concepts in your own words.) and ranking statements you paraphrased on a scale of 1-10 (10 is most important). Do enough of this tedious work, and you'll start to >>summarize<< what you're reading, and get an intuitive sense of what's most important to summarize.
1.) Search HN for "zettlekasten" and watch the youtube video about it: https://www.youtube.com/watch?v=nPOI4f7yCag
This will help in understanding that note taking is a process and what's important emerges as part of this process. Expect some inefficiency.
Pro-tip: physical notecards are still the best for fewer than 5,000-10,000 cards.
2.) Here are some tips on the physical aspects of note-taking that helped me: 1.) use a few different colors, 2.) use a highlighter for important concepts, 3.) use the Cornell page layout (see #3 below), 4.) have one main concept per page/card so you can rearrange into a logical order, 5.) summarize key points at the bottom of each page, 6.)
3.) Try the Cornell method: http://lsc.cornell.edu/notes.html https://ubwp.buffalo.edu/ccvillage/wp-content/uploads/sites/... layout explained: https://www.csuohio.edu/sites/default/files/Note%20Taking%20...
4.) You can fall into a rabbit hole of optimizing notes. At the end of the day, spending time studying is all that matters.