Why and when did you decide to start learning more math?

I have a degree in pure math. I used it once in the first 20 years of my programming career. To be honest, I was terrific at proving things from first principles, and completely unable to apply it.

Once I got into digital signal processing everything changed.

I used university-level engineering textbooks. (Make sure you work through a decent number of the exercise questions). And learn to use Matlab, or Mathematica. It's great to understand the underlying theory; but most practical applications are so much easier (and so much less error-prone) with computer-aided calculation.

Pro tip: Raspberry Pis come with a free license to use Mathematica for educational purposes. Matlab is probably more useful; but also prohibitively expensive. And Wolfram Alpha is completely free via the web, although cajoling Alpha into doing the right thing is kind of painful.