Why do I do this while my mind thinks about something else

I think it's too much pressure, and this pressure comes from academic aspects, such as grades or further education. You subconsciously want to get rid of this pressure, you don't want to learn, you hate learning. So when you're doing something that you enjoy and that isn't related to learning (like playing), your attention is focused.

This is also the reason you mentioned in the second paragraph: the pressure from the direction of study, when you do something that you don't like to do (such as washing your hair, cooking), because of your pressure, so your mind will habitually scatter to think, you should be thinking about studying, but I mentioned before that because of your pressure, your subconscious aversion to learning, so your scattered thinking is thinking about irrelevant things.

There should be many people at your stage who will have such pressure, and young people's ability to self-regulate is not very strong, and in this case, they generally cannot self-regulate. Seeing a psychiatrist can indeed accept the correct guidance and get back to a good state. But that gives you another idea:

You yourself think you're sick.

But this is actually not a disease, it is a problem of mentality, first, you don't really treat your learning problems; Learning is now your burden, your task, not your practice on the road to success in the future; These are the differences between happy and painful learning; Second, you pay too much attention to the results, and the result of your studies is a transcript and a good university. You are afraid that the result will not be good and you want to study hard, and you invisibly put pressure on yourself. But this pressure can sometimes backfire, as you do now.

Relax, as long as you really work hard, the desired results will naturally come. It is up to people to plan things, and it is up to heaven to make things happen.

Two pieces of advice are given to you: one is to keep enough rest and have plenty of energy before entering study every day.

The second is to keep in touch with the people around you. Parents, classmates, teachers, etc. Share the problems and troubles you encounter in life and study with them, and at the same time listen to their questions and troubles.

I'm not a professional, but I hope it helps.

You're like me, so there's nothing wrong with me, and I'm always absent in class, so if I play games and watch TV, I must be very focused. However, there will be one or two classes a week that I listen very carefully, and my grades are quite OK. Maybe your problem is a little more serious, it may be true that you are putting too much pressure on yourself.