I'm a bit wary of some of the suggestions here. I'm a similar position to you (at least, when it comes to maths). The difference for me is that I'm fairly certain that I'm one of the least accomplished people responding to your post. Whereas most of the others responding appear to be of the same ilk as the people you idolise. I know I'm cut from a different piece of cloth as those people (and I don't mean that resentfully - I just want to be realistic about myself). So some of the suggestions may be perfect for them. For people like me (and I'm not suggesting that you are, but your circumstances sound similar to mine), something with more of a safety net may be more realistic. I know I don't have the fortitude to sit and persevere with a text book for hours on end. Sometimes, I need some of the groundwork to be laid down for me - at least, when it comes to things like maths and scientific ideas.
I can't say I've found a successful way of learning and retaining maths knowledge over the past few years. That retaining bit is important - I can pick up something, give it a go and get it right, but if I don't do it again, I forget what I've learnt. I had some early success with the OU book series 'Countdown to Mathematics' (https://www.amazon.co.uk/Countdown-Mathematics-1-v/dp/020113...), but the problem ultimately is that it covers materials in a block format; once you've covered a topic and solved problems, you move on to the next topic. Khan Academy has the same issue and is arguably worse because, so far as I've found, it requires learners to know what they don't know, rather than structures a learning programme where each topic follows sequentially (maybe there is a way to do this but I've found the KA UX to be complicated and confusing).
I've tried more 'sophisticated' maths learning solutions that claim to account for learners' knowledge and weaknesses, but there are various shortfalls with them and none is aimed at learners older than schoolchildren. It's not so much the provision of learning materials that I'm frustrated with but the process of testing and retaining what I learn. I would dearly, dearly love to find a tool that can assess what I know, tell me what to learn next, test me on the topic BUT continues to do so as I progress, making use of spaced repetition and interleaving. It seems like the perfect use of both techniques - like Anki - but so far as I've found, nobody has done this for maths learners. Anyway, I've digressed...
I can't say I've found a successful way of learning and retaining maths knowledge over the past few years. That retaining bit is important - I can pick up something, give it a go and get it right, but if I don't do it again, I forget what I've learnt. I had some early success with the OU book series 'Countdown to Mathematics' (https://www.amazon.co.uk/Countdown-Mathematics-1-v/dp/020113...), but the problem ultimately is that it covers materials in a block format; once you've covered a topic and solved problems, you move on to the next topic. Khan Academy has the same issue and is arguably worse because, so far as I've found, it requires learners to know what they don't know, rather than structures a learning programme where each topic follows sequentially (maybe there is a way to do this but I've found the KA UX to be complicated and confusing).
I've tried more 'sophisticated' maths learning solutions that claim to account for learners' knowledge and weaknesses, but there are various shortfalls with them and none is aimed at learners older than schoolchildren. It's not so much the provision of learning materials that I'm frustrated with but the process of testing and retaining what I learn. I would dearly, dearly love to find a tool that can assess what I know, tell me what to learn next, test me on the topic BUT continues to do so as I progress, making use of spaced repetition and interleaving. It seems like the perfect use of both techniques - like Anki - but so far as I've found, nobody has done this for maths learners. Anyway, I've digressed...