I fully agree, and especially for mathematics the key is definitely practise, practise, practise. That's why it's so hard to learn mathematics without a teacher

I fully agree, and especially for mathematics the key is definitely practise, practise, practise. That's why it's so hard to learn mathematics without a teacher, it's all about exercise and you really need someone to explain to you how to solve a certain problem when you're not able to solve it for more than 3 days or even weeks. If you know someone who can explain yet-unsolvable problems to you, then you can get far with self-learning, but my overall experience is similar to yours. I've got books on machine learning, physics, and all kinds of interesting topics, but as long as I don't have the time and energy to seriously work through all the exercises, they will at best only give me cursory overviews of what's going on in the field and what I could learn. What you can do yourself is to get into new domains once you already have some solid background or to check papers and theorems that use math you already are familiar with. But that's not learning math, of course, that's just applying existing skills.