No, you don’t become a kafir. However, you should expiate it by feeding ten poor people.

long ago, i made an oath if i commit a certain sin i will become a kafer. i asked you about it and you told me such an oath is a major sin. so i regretted making the oath and repented from making the oath altogether. now, if i ever commit that sin, do i become a kafer and have to take shahadah again every single time?