What is the best way to learn without thinking ahead? I've run into this issue in many fields in which I'm interested in learning about. For the purposes of this question let's assume electricity/circuits/power systems. I find myself starting at the basics and as I try to understand each concept, I start thinking of more of how this concept really works instead of accepting it at face value. In addition to this I also start to ask more complicated questions which I'm assuming I could answer if I would read on. The problem I end up running into is I won't let myself continue until I understand the concept completely. For example: I start learning about how current is always consistent in a single circuit which makes me start thinking about how total current/draw is measured in a household. Then this leads me to think that if everything is connected by wires, then shouldn't this be one large circuit with the same current.

As you see I start diving too deep. This seems like an inefficient way of learning and it usually causes me more stress than understanding.
A good way to force yourself to focus is to try to actually do something with the knowledge that you are trying to acquire.  To paraphrase one of my former advisors who I much admire as a teacher, there are three levels at which you can understand a subject:


Familiarity with the concepts: able to recognize and talk meaningfully about them.
Ability to employ the concepts effectively in solving problems
Ability to teach the concepts to another person well enough to get them to level 2.


It's easy to get stuck at mere familiarity if you aren't attempting to actually use the things you're learning in order to do things.  From a pedagogical standpoint, this is actually the main value of homework exercises.  

Now, what about doing it in the absence of an external forcing function (i.e., grades)?  If you can't motivate yourself to do homework-style exercises, why not pick a project that you actually think would be cool and fun, and let that be what forces you to put the learning to work.  For example, taking the electricity/circuits example, a nice classic example is to try to build your own FM or AM radio receiver from basic components (transistors, resistors, etc.).  It's low enough power to be physically safe and complex enough that you'll probably have to deal with a lot of interesting problems even if you start from schematics found online.  Then try to make it better... before long, you'll understand not just the basic concepts but why you need to know them and how they fit together.