Since you can understand the class, it is not a problem. Just do more questions.

It's just a matter of proficiency.

Often because I am not interested, I don't want to do this kind of exercise at ordinary times, and I don't want to think about a slightly more difficult problem, so I don't think so.

Start simple and gradually increase the difficulty. When you keep doing a lot of problems smoothly and the difficulty is increasing, you will have a sense of accomplishment and become addicted.