This really can't be emphasized enough. I repeatedly fell into this trap of thinking, "If I just master this one more discipline, I'll finally know enough to build what I want". What you end up with is a daunting pile of textbooks to read and no real progress. It's a very insidious form of procrastination because it seems like a good and worthwhile thing to do. Of course, it IS good and worthwhile to learn new things, but learning alone doesn't produce any tangible results.

In the end, you can get a lot further than you think by just puzzling things out for yourself. Try something, see where it fails and adjust as necessary. You'll learn as you go and you'll have the advantage of knowing that what you learned is actually useful knowledge, because it solved a real life problem you encountered.