Boredom sets in when the challenge becomes too easy.
Are you working through a few textbooks on the same subject, at the same level? If you've already mastered a subject from reading one textbook then it's going to be really boring reading it through again in a second. Learn to skip!
The same goes for doing problems. Many textbooks have dozens of simple problems that you can do easily, so there is no challenge, and you quickly get bored. But many will say, and they have a point, that it's useful doing many problems to really hammer in the understanding.
So - learn to skip easy problems, but seek out harder problems in other textbooks & past exam papers. (Don't just skip and play video games!) Of course, now you face being totally stumped by some problems 'cause the challenge is too high, and this leads to stress rather than boredom. So - pose these hard problems in these forums (or any other help!), and while waiting for answers, go and look for slightly simpler problem sets!
I think you've found one of the main problems of self studying. In a University, good lecturers arrange a course of study that should keep boredom & stress at a minimum; with snazzy lectures, directed reading, exciting tutorials, instant help, superb labs, useful exams, and problems sets that are at the right level of difficulty.
You, as a self-studier, need to be your own director of studies without knowing the subject. I'm not sure this is even possible, which is why there aren't any self made physicists around today, at least not any that I've heard of. (If there is, they need to write a book on how they did it!)
Physics and maths textbooks are infuriating! You start reading a chapter and they draw you in with interesting concepts; it becomes even more interesting than reading most novels. But then they introduce a concept you can't understand (stress!), then they give you lots of tedious problems (boredom!). You try another textbook and it just increases the web of interest, stress and boredom. It's difficult! You need a support group. So take a course, don't self study. Or give it up and read novels, or take up fishing, anything that holds your interest which is less traumatic. That is, anything but hard drugs.
I keep on thinking of self studying some advanced physics (I did a degree in physics many moons ago) but the hurdles are so high, the path between stress and boredom so narrow, I think I'll just stick to reading novels, and surfing the forums and chatting.