It's actually your psychology. You will always find this set of papers difficult because of a problem, and there is always a feeling that there is not enough time. Actually, there is no need to worry. I have done experiments for my students. A seemingly difficult problem takes the same time as a difficult problem. What you need is to learn the balance between gains and losses. Just because you have a problem doesn't mean you can't, it's just that you haven't figured it out yet. Later, the teacher said, sometimes you must feel so simple. I just didn't think.