I was sort of the same way. One thing to keep in mind there is no "mastery" in many skills out there. There is a level of expertise that you can get casually an increasing level of expertise based on the effort and time invested.
The way I focused was rather simple in hind sight. I basically made a list of things I wanted to "master". Over a long weekend I decided which one skill I wanted to focus on. For me this way a career motivated decision so my benchmark I had to achieve was being skilled enough in that trade to qualify as a "professional" (IE I could get a job in that field and feel confident I could perform my duties to at least an acceptable level)
As I got closer to that goal I found myself shifting the goal further down the pipe. I'd have to qualify as an intermediate, later a senior in the field, etc. My new problem is... okay... While their is plenty more to learn, I haven't been able to come up with a new goal other than "learn more" (it's a very satisfying problem)
TLDR Choose a skill, set a goal, don't quit until you get there.