The energy and interest you have is a tremendous positive. Don't take it for granted. It's something to nurture. What exactly is making you feel bad - what are you thinking when you have that feeling? i.e. what thought is stopping you and why is that bad? [OK, I'm guessing in the following - you'll find the true answer in the above questions] You say you feel bad because of a lack of mastery, but what level of master…
What the OP describes is a system which he acknowledges improves his expertise in the studied knowledge.
My opinion is that system has the advantage over goals in that it is an automatic and permanent drive to stick to some rule. It can be learning, exploring new ideas, do fitness, healthy food, keep a good weight, etc.
Goals require a boost of effort and focus wich can only last for a limited duration. You feel good if the goal is achieved and bad if not.
If you work with goals and sprints, there is a high risk of burnout. So better go into system mode for the long run.
The change of topic of interest is something I saw my self and was also puzzled about. I attribute this to some "fatigue" of the brain where it's learning capacity is kind of saturated. Changing topic of interest, changes the neurons and connections in play which let some of them to rest and activate others. Learning is also a matter of repetition and pruning details. You get this by switching back and forth between topics and forgetting the details in between.
Another benefit of having different topic of interest is that at some point we will make the connections between them. We'll use the knowledge gained on one topic on the other. This is powerful and by the combination of topics explored it will also make you most probably unique.