Remember: the cultural heritage of America isn't only white supremacy, sexism, racism, bigotry, and capitalist greed. Our heritage is ALSO abolitionists, labor unions, civil rights, women's suffrage, environmental activism, etc.
Don't look at history just to find the mistakes. You can also look at history to find the victories and inspiration. We stand on the shoulders of generations of people working together towards a more perfect union. Sure, there's a whole lot of shit that's fucked up, but there's a whole lot of people who are also going out there every day to try and fix stuff too.