I want to study history, but there are too many details in the books. and i get lost in these details. For example, I am currently trying to read the middle east book by bernard lewis, but I am making very difficult progress. Should I research everything he mentions or just read superficially? can you give advice?
The goal to study history is an extremely broad desire. I would love to help you, but I'd need more info:
What setting do you want to/are you studying in? That is, do you want to study for school? For a career? Are you just dedicating time to reading a book during downtime?
What is the ultimate goal? Do you want a job in field - academic or otherwise? Are you learning to enrich yourself?
Do you have a region or time period you want to study?
General advice:
It is IMPOSSIBLE to know everything. Even the most accomplished emerita are not all-knowing beings ascended to godhood. Accept now that you will always be a student of history.
Developing good reading habits is what makes or breaks a lot of folks trying to get through school, particularly at the graduate level. This is an especially useful skill if you are learning for fun: learning to filter what is important to you and what is basically fluff is a way to specialize yourself and actually enjoy what you're reading.
I don't exactly know if learning is ever "superficial." If you have a stated purpose to learn, you are actively working towards a goal. If your goal is to become a professor, to write a book, to pass a class, or to enrich yourself, you are working towards all of those things by reading and learning.
EDIT TO ADD: Please tell me how candid you'd like me to be in crafting a response. I don't want to be too harsh but I also want to be realistic depending on what exactly it is you're after.