In my case, any science would be difficult. However, it all depends upon the student. Some students naturally gravitate towards science and can understand it quite easily. However, there are other students like me that can’t. I would believe that compared to the other sciences cosmology would be one of the most difficult because it is the study of the universe and the exploration of the universe is quite new.
It wasn’t until recently that people were capable of going into space. Other sciences like biology has been around probably just as long. However, scientists can easily study animals because they have always had access to them. It would probably take a lot more studying to pass a subject like cosmology than other subjects.
It's tough. You need to be a lateral thinker, almost a philosopher, as well as a keen and rigorous scientist. It is not perhaps as romantic a subject as you might think, studying the stars and distant realms of the universe. How broad a canvas can your thought process contemplate? You need to feel comfortable in the realm of quantam physics and open to delve into areas you had not thought of before.
Why not look at the biographies of cosmologists as well as the content of university courses? Altogether, starting a three-year couse in cosmology is of a different order to, say, one in Geology where the boundaries of the subject are more defined.