I recently faced the same realisation. I'm a software engineer with several years of experience and somehow felt like I didn't know enough about formal computer science/math to tackle the problems I found most interesting (usually very abstract, foundational stuff).

One day I decided to go to a physical bookstore and buy a bunch of books from the Math and Computer Science sections and started from there. It was probably not the best way to start but A START nonetheless. Given that I really enjoyed reading about these topics, I decided to enrol into an online university to pursue a degree in Mathematics.

The book I found the most useful was "How to Prove It"[0]. From my point of view, it was a great starting point for two reasons: * It is approachable (specially for Software Engineers) without being boring. You start building intuitions and it really ignites your curiosity. * It is also sufficient for understanding proves and mathematical notation/language. This is an important building block that will allow you to start tackling the branches in Math you are interested in.

[0] https://www.goodreads.com/book/show/739735.How_to_Prove_It