Read other people's code. There's a world of high quality (and not so high quality!) code out there - pick a reasonably simple open source app and read it's code. Think about how it's put together: Why did the programmer chose to cut the problem space into those particular pieces? How do those pieces interact? Is the API obvious, or completely opaque? Is it completely batshit internally? (I've seen some batshit code in my time. I even wrote some of it.) How could it be improved?

Mentoring with a senior dev is a great idea if you can find someone willing to sit down with you, but just doing the work of going through real world code is well worth your time IMO.