I was in the same position and did some research to get better and I seriously recommend starting with Gelfand's book on Algebra (and another Gelfand book on Tr

I was in the same position and did some research to get better and I seriously recommend starting with Gelfand's book on Algebra (and another Gelfand book on Trigonometry. These are educational books developed by one of the best Soviet mathematicians translated perfectly to english. It starts with the absolute fundamentals of Math, but asks such critical questions that you develop a much more nuanced understanding of maths you thought you already understood. From there I plan to slowly make my way through the entire Gelfand library and Spivaks Calculus. Maybe down the road Category Theory and Engineering Maths, or Physics. There are so many amazing resources online that I don't see any reason why a motivated learner with spare time can't achieve an education on par or better than a University undergrad.