It's a hard subject to teach yourself. I was in a similar situation to you, and after starting with various books I ultimately ended up enrolling with the Open

It's a hard subject to teach yourself. I was in a similar situation to you, and after starting with various books I ultimately ended up enrolling with the Open University for a BSc in Mathematics. Before that though, the most useful thing I did was to get the OU course books for their introductory courses (MST124 and MST125). They're written for self-study, and they're extremely comprehensive. These books filled in all the gaps and ironed out all the kinks in my fundamental mathematical knowledge. Things that used to baffle me make perfect sense now, like why the rules for non-integer exponents work the way they do. Another book I'd recommend for someone in this situation is Measurement by Paul Lockhart. It won't exactly teach you maths, but it'll teach you what maths is, which is an important thing that most people are missing.