It's called the monkey mind. I recommend with-holding the reward of starting something new until you've completed the first course.

As you've noted starting the a course and not finishing means you haven't gained from the course and cannot say that you've learned.

One thing no matter how long you've left the course, continue from where you left off. The false starts just keep repeating. Do the next CD lesson from where you left off. And aim to finish it.

Starting over is boring. You've heard it and probably experimented a bit with what you learned already. And it's a sure fire way of getting stuck again.

The fact that you are continuing from where you left off you give yourself space to not get it Not fully understand it and just take what you can from the course. That way too you can decide whether you do want to review any parts of the course again. You at least did the whole course before the monkey mind pulls you to the next one.