I've been slowly working my way through Introduction to Calculus on Coursera.
I was initially shocked at how much basic stuff I didn't know. Maybe because I was homeschooled and finished two years early than the full curriculum, to pursue career and money (which I don't regret BTW). But it seems I somehow forgot or missed super basic stuff like surd expressions, or how to do long division, or certain details of the order of operations (yikes!), or the fact that squaring a negative number makes it positive.
The nice thing is, after swallowing my pride and working diligently and rigorously through learning all these basics, I'm starting to really understand the material and it's a delight! I'm up to Week 3 of the Calculus course now and regularly getting 80-100% of the practice questions right on the first attempt.
I feel like I'm learning the doing of math, but also getting a firm handle on the concepts. This is a much better learning experience in adulthood than I could've possibly had if I'd tried to rush through this stuff back in my teens.