You sound as if you are already able to teach yourself, just read more open-source code to find out what is good or bad style.
Sounds like there should be a thread to match mentors and mentees here on hackernews. Maybe a bunch of mentors post some open source projects that they want mentees for?
Every time I try to 'contribute to open source projects' online I get overwelmed by the size and complexity of most codebases, I have tried numerous times, and have never been able to fix a bug.
This type of thread is something I'm sure I and others would take serious advantage of.