>I've tried Khan Academy

Did you try the exercise dashboard on Khan Academy? https://www.khanacademy.org/exercisedashboard

I've tutored math for the SAT. My anecdotal impression is that many people missed a few bits of math in the early grades, then spent the entire rest of their math careers hopelessly behind.

So, for example, algebra might be difficult because of a gap from grade two about not knowing part of a times table.

If my hypothesis is correct, that would call for approaching math from a total beginner mindset, and taking nothing for granted. For example, there is much to contemplate in a triangle, or in mental tricks for adding small numbers.

Practice is important. That's why I like Khan: for every concept, they give you exercises you can do to the point of mastery. I believe it also does review exercises so you keep working on the knowledge.

A sense of play and fun is important, and helps with practice. My dad is excellent at mental arithmetic: when he sees a license plate, he'll add the numbers together for fun, or multiply them, etc.

Another app I like is Dragonbox. I'm not sure how far it will take you, but the goal is building intuition. Again, beginner mindset is important. It's intended for kids, but for all intents and purposes you are on the same level as a kid – and that's fine!

https://itunes.apple.com/app-bundle/db-complete-math-pack/id...

Another important element is that people who are good at math have usually just figured out small tricks that make certain calculations easier, and help them visualize concepts.

I'm not entirely sure how to teach them, but while reading Isaac Asimov's autobiography I came across this reference to a short book he wrote on exactly that topic. Asimov is a wonderful writer and this may be an excellent primer:

https://www.amazon.com/Quick-easy-math-Isaac-Asimov/dp/B0006...

Finally, I recommend studying for the SAT math section. The questions test math in somewhat novel ways. They're fun and reward creative thinking. And as a bonus, there are books written to help you figure out this kind of math, quickly. I really, really like Pwn the SAT. My students got a lot better just by using it: it teaches you how the author thinks, and really breaks things down simply.

SAT practice tests: https://www.amazon.com/Official-SAT-Study-Guide-2016/dp/1457...

Pwn the SAT: https://www.amazon.com/PWN-SAT-Guide-Mike-McClenathan/dp/152...

(Note: The SAT changed in 2016. You could also use the earlier SAT test booklet (the blue book) and the earlier Pwn the SAT for this exercise)

I don't think any single concept in math is particularly hard. What's hard is that even high school math requires mastery of a few hundred concepts. So you can learn one, but it goes away.

The tools I outlined above are the best I know to solidify math knowledge and make it habitual. I believe any reasonably intelligent adult could use these to learn math.

It is, however, a large subject, and I expect it would require months of focussed work practicing these for every day. But I believe it's doable. I'm basing this on my experience with students who were "bad at math". They made great strides.

I hope this may be of some use! Feel free to reach out if you want to talk about it further. My email is on my profile.