In my experience, it's vagueness of learning goals that actually kills my attempts at learning.

In an effort to overcome this, I now try to commit to a formal learning program with examinations and tests for things I want to learn. This gives me a definite body of knowledge to master and a criteria to judge my progress. I have used this successfully with learning the rudiments of Sanskrit. I enrolled in a correspondence course program and had online sessions with a Sanskrit tutor for some time.

Where such a program doesn't exist or isn't feasible, it still helps to stick to a definite text. For example, if you want to learn classical mechanics, it helps to stick to just one book rather than refer to books and videos from here and there and losing track of your learning program in the process.