Hello, I'm 16, in the UK. I've always struggled with maths in school despite enjoying it in other areas (when programming, and in videos by ThreeBlueOneBrown etc). Exams tend to be alright because I can problem solve, but I've never found it easy.
My 10 year old sister's been having a similar experience learning fractions. I sat down to help her with some homework and quickly discovered that instead of teaching her what a fraction is, she's learnt a series of heuristics that get the right answer for a given question while not understanding what she's actually doing. I started to teach her what fractions actually are - to the level of "Fractions are parts of something", "1/2 means one half of a whole", "the bottom number means how many parts the whole has been broken into, the top number how many of those parts you have" - etc. It finally clicked for her, so we decided to go over her assumptions regarding decimals as well: Again, they'd taught her "zero dot twenty-five means one over four", instead of what they actually were. 20 minutes of explaining place value later, she's gotten that too. By teaching her what things actually are, I've gotten her past months of falling behind a class as she struggled to learn the heuristics. I'm not a particularly good teacher - it's just the way it's been explained to her that caused issues.
That experience prompted me to think about my maths education, and I've had the same issues - e.g. I've learnt several rules for computing trigonometry, but I have no idea what sin, cos, and tan really are. This post is getting long so I won't go into more details, but I think I would benefit from resources that teach mathematics that way - teaching what things are, not just how to manipulate them through tricks. I'm starting maths and further maths A levels in a few weeks. I've looked at resources like ThreeBlueOneBrown, and while the quality of the content is incredible, he doesn't cover anywhere near the breadth of content I'll be doing[1]. I would really appreciate recommendations for more content like it, that explains secondary (high) school mathematics using the why.
[1] https://filestore.aqa.org.uk/resources/mathematics/specifications/AQA-7357-SP-2017.PDF
Another approach that I recommend to people all the time is the Open University. Enrolling in a course is obviously a pretty big deal because they're expensive, but you can usually find used copies of the books on eBay etc. I'd recommend MST124 for the kind of stuff you're talking about. They're hands down the best, most comprehensive maths books I've ever seen. They're written for pure self-study, so you'll rarely have questions that aren't answered by the text or by the solutions to the exercises.