graduate school - Lack of motivation for self-study during the semesters

I'm a third year Physics and Mathematics student, and over the years, I've done lots of self-studies, and what I've realised is that, compared to what I learn & experience in my self-studies, the I barely learn anything from the lectures that I'm attending my departments (at least this is true for %95 of them).

However, the thing is, during the semester, there are lots of midterms, homeworks, and lectures to attend etc.. Hence, for most of the time, even though I've time, I cannot find any motivation to continue to study to the subjects that I'm self-studying. Which is not the case in the summer and in the winter breaks; at those times, I generally do not have any problem about the motivation, and I study with quite normally & happily.

Seeing that professors also do most of their research during the holiday seasons as I do (since during the semester, there are lots of teaching to be done, exams to be grade etc.), is such a lack of motivation general in the academia ?

I think if there was no courses, during a whole semester, I would learn more things & deal with more complicated problems, which would require me to think more; however, right now, the things that I have learn are quite boring things (which honestly if you just attend the lecture and listen, you can do it), but these requirements are kinda of pushing me away from studying, as if I'm doing this just because I have to, and not because I think this is interesting.