"You see there are three Americas, there is North America, where you live, and there is Central America where I'm from, and there is South America where many people live. We are Americans also, not just you. We call ourselves by our nation, I am Guatemalan, some people are Bolivian, or Brazilian, but they are all Americans too."
We quickly entered the building we were walking toward and I never had a chance to even say "I know, I know! I'm not another ignorant American!" It was a busy day and I let her go and never followed up. But here is what I would have said to her:
"See here Polly Perfect, we use "American" because America is in the name of our nation. Is America in the name of your nation you hyper sensitive Guatemalan smarty-pants? None of the other countries in our hemisphere use the name in their titles. So when someone says I'm an "American" it's actually very well implied because they said nothing else to identify a different nation otherwise. Bolivians in Bolivia don't say I'm a "Bolivian American," but Bolivians in the United States of America do say so. It's because of our name, and despite what you think it's not egotistical jingoist behavior you Guatemalan American nit-picker! Now go find something constructive to complain about and carry with you on your Guatemalan American back!"