I hope this question is appropriate for MSE.
The situation is that someone is reading a book (e.g. a monograph), possibly helpful in his/her research, and the content is sufficiently extensive or deep. At first glance it seems very frustrating and time consuming. At least, I feel a bit of frustration.
I want to know how mathematicians keep themselves motivated and achieve this task? How do they manage their time to learn new concepts, especially when someone is a beginner researcher or Ph.D. student?
I will propose an answer just so there in one!
For me it helps to ignore all literature and go for a walk. I ask myself what bothers me, what do I want to know and what are my interests. Then I go back to the library and look for an answer; without trying to understand how or why. I merely try to connect two dots, even if the method is completely stupid. If I find that answer then my interest (see start of story) gives me the motivation to understand how the text got to the result. If at any point I am frustrated or bored I simply restart my process. I go for a walk...
Finally my algorithm is totally personal to me but it does work for me. Others may substitute walk for watch a film. Anything to re-discover your childish passion for learning. The key to success is having that passion (see numerous famous people who have said similar).