I'm not quite in the situation you're looking for, because I have got an undergraduate maths degree, but due to my personal situation at the time, I wasn't a good student and I have done a lot of self-teaching in the years since then. Mainly in mathematics, but also theoretical computer science and some programming.
One thing I'd say is don't overlook books. Textbooks may seem less appealing than slides or videos, but seriously sitting and reading a maths book from cover to cover is a very powerful experience. It may take months, but you will learn something significant from doing it. And if you do that for say Thomas's Calculus and Anton's Elementary Linear Algebra, that's about half of a maths degree already.
Anyway, I'm happy to discuss these topics further with you (or anyone else reading this). I'd personally love to find a serious online community for studying mathematics, but it seems very hard to coordinate such a thing.
