I found that another hard thing that feels like a prerequisite for leveling up further is getting the feel for design/architecture of medium-sized systems. There's a pretty awesome resource for that particular need - the series of books called "The Architecture of Open Source Applications".

http://aosabook.org/en/index.html

(The books are CC and free to read on-line :))

I'm just through the few first chapters of the first book, and I must say it's absolutely amazing. Each chapter gives some understanding of the thought process people designing (and iterating on) a known open-source project had.