Hi Mahdi.
1) "is the question bank of quiz different from a question bank of the mock exam?"
Yes. The question bank for both the quiz that you do per knowledge area or domain is the same as the one you use for the mock exam.
2) "after reviewing each chapter of PMBOK I run timed quiz ( 50 questions) of related chapter but unfortunately, my results are between 50-60 %, could you give me advice how I can improve?"
Please review the questions for each of the exams you do. Start with the questions you got wrong, then the ones you marked for review (whether wrong or right) and finally the ones you got wrong. Reviewing the questions ensure that you understand the concept and the reason why your answer is wrong. This process takes a lot of time and is very boring and almost feels as if you are making very little progress. However, this is a sure way of passing the PMP exam. It takes hard work and a lot of seemingly "boring" work to ace the PMP exam. I sure hope that helps.