1. You are not proficient in solving the problem you want to do. Practice makes perfect. When you are familiar with it, you can easily solve the problem according to your own feelings. If you are unfamiliar, you need to think a lot and be cautious, but you are prone to mistakes.
2. The problem of concentration, when most people concentrate on something, such as an exam, there will be a distraction in about 15-20 minutes, if it happens to appear when you are thinking about a question you are not familiar with ... but it belongs to a kind of ability, called concentration. Geniuses in history are concentrated, such as Einstein. You can check their stories.
There are but not many solutions:
1. Practice more and correct more mistakes: because of the basic questions, you may need a lot of practice to correct and summarize the mistakes. Only when you are familiar with what you have learned can your accuracy be significantly improved.
2. Concentration training: concentrate on one thing as much as possible, repeatedly record your concentration time, remove distractions, eliminate external interference factors, and improve your concentration time. Meditation is good.