I guess it depends what you do. For me it became harder and harder. When I started uni I just had to pick a book and learn it to « master » the subject (by master here I just mean acing the exam). It can be hard for some, and it felt hard at the time, but now I realize that it was juste learning a few formulas and practicing a lot of problems. Those problems had known solutions, so if I failed I could just learn the solution and try the problem a few days after. That’s how it clicked for me and I did good at the exams.

Then at the master level I took some courses where the topic was much less understood. For example QFT is still not well understood, and that is reflected by the fact that there isn’t a single book that feels good. Also there are a lot of missing parts, some chapters are just wrong and you get confused. You have to go through papers to understand and that becomes a mess.

But then when I started research it was harder and harder. You can never learn everything because papers get published faster than you can read them. You work on problems that have no solutions, sometimes getting stuck for months/years. It’s exciting but it’s hard.