Firstly, don't use those books that suggest to teach you more than one subject at a time, in a traditional way. In mathematics those kind of books do not work, at least in my honest opinion that is. What I mean by this is that, you will have some knowledge over the subjects taught by the book but you will have no understanding what so ever, or it won't be good enough in the long run.

As you said, the problem was that you didn't pay any attention while in high school. It was because you had no interest, and if you try to self-learn mathematics the same way you tried learn basic maths in school, then you will also fail.

The answer to all this is better books or a different kind of approach. I should mention that this is all from my own ongoing experience. There are traditional books that cover high school math[0]. And there are bad ones and good ones. The good ones still throw definitions and theorems at you, but it's more clear and concise, and most importantly understandable.

Now comes the new kind of books which try a different approach to the subject at hand. They try to give more understanding that anything else. I only read "Burn Math Class" by Jason Wilkes and "Math, Better Explained" by Kalid Azad. These books lack exercises, which are, in my opinion, as important as understanding. But one doesn't work without the other.

As for the future just follow this[1] so that you won't get information paralysis or other difficulties that come when there are a lot of choices.

[0] https://www.physicsforums.com/insights/self-study-basic-high... [1] https://www.physicsforums.com/threads/micromass-insights-on-...