I am stupid, I am stupid, I have no brain, I am stupid, I am not agile, I am slow, how can I become smart?

First of all, we should establish self-confidence and don't give up on ourselves because of temporary disappointments. You know, only when you respect yourself will others respect you. Secondly, exercise your logical thinking ability, read some books in this field, or do some exercises to form good thinking habits, and you will do things regularly.