In American English, a person from the United States is called an American.
While this may irritate Canadians, Mexicans, Colombians, Brazilians, and everyone else that also lives in North America or South America, the word American is almost always used to refer only to people, places, and things relating to the United States of America. Sorry, but we didn’t make the rules!
“United Statesian” is not a word. North American typically refers only to the United States and Canada, not Mexico, although Mexico is geographically in North America.
This is likely because the United States and Canada are primarily English-speaking cultures, while Mexico is a Spanish-speaking culture. Thus, Mexico is often grouped with the rest of Spanish-speaking Central and South America (except for Brazil.)