Data structures and algorithms. I don't have a formal education either but after 25 years of programming, IMO everything boils down to data structures and algorithms - the former more so than the latter. I think everything else comes from experience gained from on the job training because you need to train the pattern matching part of your brain. A good foundation in data structures and algorithms simplifies that process a lot.

Data structures are how programmers describe reality in the Wittgenstein/Sapir-Whorf sense and they shape our understanding as much as we use them to encode it. The right data structures drastically simplify problems and the wrong data structures turn them into nightmares of bugs and spaghetti code. Database schemas are the natural extension of this at the application, rather than module, level.

I don't have a specific recommendation because data structures is such a broad field, but an MIT OCW data structures and algorithms course is a good place to start. Oh and the book Designing Data Intensive Applications for a higher level overview of how data structures and algorithms are used in the field.