I am self-taught and have been working as a software engineer for about a decade. I have this feeling come up a lot as well. I miss the days of when I was working on projects for myself, learning new stuff constantly, spending hours trying to figure something out and the mental rush you get when you finally get something working.
Working in software development as a profession does take some of the joy out of that over time, but I've found that trying to learn things outside of working and working on my own projects is still very enjoyable.
I have done courses on udemy like this before for learning new languages and tech stacks. There are plenty of courses on there that you can usually get for $20 that will teach you by building a bunch of small apps or services that build up in complexity.
This was the first course I did on Coursera and it is by far the best introductary course for machine learning I have ever seen, but my sample size is pretty small ;)
Working in software development as a profession does take some of the joy out of that over time, but I've found that trying to learn things outside of working and working on my own projects is still very enjoyable.