I am looking for (free) resources for semi-advanced math.

I have always been extremely mathematically-inclined, working with material way ahead of my age, then I went on to study electrical engineering. I got a master’s, but I feel like I stagnated severely during my studies. It was then that I first hit my first walls - particularly, I remember Bessel functions being beyond me back when I studied anthena theory. But I also breezed through all of calculus, including multivariate, vector and complex kinds. However, after graduation, I barely need any kind of math more advanced than Laplace tranforms done with a handy table, and even then I start to forget a lot of things.

I think I might have had more potential and would like to get deeper, but I don’t really know where to start. I am in my thirties and I don’t want to forget math, which has always brought me a lot of joy.

Where do I start?