Maybe they become bored with you because you're just fucking boring, and you're like the last 27 guys she has gone out with.
If "most american women" are the same, wouldn't it logically follow that "most American men are the same" too?
I'm sorry to use you as an example, but you've shared your experiences here so they are discussion fodder. You've got to ask yourself: what is so great about you that a "typical American woman" would act differently around you?
Until that question has an answer, you're never going to solve this dating / relationship riddle.