I believe that every mathematical "apprentice" more or less once found that some stuff that had been unclear somehow became clear; for example, some books might be unaccessible to you two years ago but totally accessible now. Even having these experiences, from time to time I would still be tempted to think that I may not be able to go further in mathematics when facing those now-unaccessible stuff. Now my question is how you develop a good mindset to turn away from the time-wasting behavior of doubting your own ability?
This question is possibly opinion-based, in all conscience; but it depends on people's flavor.

I guess we all encounter some "heavy" material from time to time and feel pressure for not being able to understand everything perfectly. This is perfectly normal, or to rephrase it in the immortal words of von Neumann:
Personally, as long as I can remember, I've done maths and I've been good at it. I've decided to study mathematics when I was 10, or something like that. I am far from being very talented mathematician or very hard working one. I simply love maths and for that reason alone I've always felt that there is nothing in mathematics that I won't eventually be able to grasp. So, when I feel frustrated about a subject I'm trying to learn, it's only temporary as I'm aware it will become trivial soon enough, just like all the stuff I learned before.