If you want to stop wasting your time, and want to learn something thoroughly and once, I would recommend learning how to use Anki effectively. It has reduced the amount of time it takes to learn new things, while simultaneously allowing me to remember them for far longer periods of time. I can currently stop an online course for months, and return to the same point when I come back, with far more knowledge of the subject than I had when I left.

Anki, like using a calendar and communication tools effectively, is just pushing the burden of organization, memory and attention out of your head, and into your environment. This will not only save you time, but it will also, if set up right, give you that sense of peace of knowing that whatever you are doing is exactly where you should be.

It's solid. I learned about it in "Learning How to Learn", but the mental concept stuff is from "The Organized Mind".