I am surprised that no one has mentioned consultation with a MENTOR. Really, if you "don't know what you don't know", you need some type of trusted guide who understands your aptitudes and motivations and can recommend a path of study or provide some clarity of thought. There is absolutely nothing wrong with fumbling around in the dark and discovering stuff on your own, but if you're asking this question, it means yo…
Unfortunately where I work there aren't any people that would fit that bill. Finding a mentor outside of work is difficult for many reasons.
From what I've found this is a pretty common problem.