Self-education left out many fundamental facts for me [on hold]

Self-education left out many fundamental facts for me [on hold]

So I've been programming/building web sites for about seven years, started
when I was 12 with HTML and then a few years later I got into Java.
Everything I have learned is from the Internet, at least 99 %. The problem
with self-education is that you miss out on important things in order to
fully understand why certain things work and other things don't work. I
mean I only know the stuff that I have had to learn in order to build my
games or websites or whatever project I've been working with. For
instance, just last week I learn why an int can't be null while an Integer
can (in Java). And this is very fundamental stuff that you probably should
know way earlier in your career than after seven years. (This was just one
of many examples.)
So I guess my question is, how do I learn these fundamental stuff, without
having to go through beginner's books and articles that teach basic
programming that I already know. I'm not really sure this is a good
question that can except proper answers (didn't even know what tags I
should use for the question). However, now that I'm beginning with more
advanced programming I feel that my lack of some fundamentals in for
instance Java is really holding me back.