I skipped two years of school which also happened to be when a lot of the basics of Algebra were introduced. When I returned to school my scores were good enough to not repeat but the gaps didn't become obvious until it was too late to fix.

Reading that article I now realise it was that I lacked fluency. I didn't instinctively "know" how to do simultaneous equations because unlike my peers I hadn't spent two years doing them, so I had to remember how to solve them every single time.

All I can say now is thankfully there is the Khan Academy which rapidly improved my mathematical understanding when I needed it.