[dmfdmf]

Math is a hobby of mine and, as an adult, the best way to learn math is by trying to solve problems that interest you, i.e. for fun.

I'd recommend getting a copy of Mathematica. The home/hobby version is only $376 for the desktop version though you could start on the cloud version for less. MMA is great for checking your algebra (the bane of my math work) and graphing and using it for "what if" questions.

Also, hang out on the math and mma stackexchange forums and read posts and when something interesting strikes you dig in. Years ago I discovered that the lemniscate is the inverse transform of the hyperbola. It wasn't a formal proof but close enough and I was pretty excited about discovering something like that on my own. Also, check out John D. Cook's and Daniel LeMire's blogs for interesting ideas to investigate.

Enjoy!