To be clear, I am not saying that Native Americans "just need to get over the past atrocities committed against their ancestors". I understand why these things bother people. They probably should bother people. They were a horrific mistake and should be a lesson on what not do going forward. I am not telling anyone not to feel upset or bitter.
All I am saying is that, for me, America is more than a collection of mistakes and times where we've failed to live up to our ideals. Sometimes, we get things right, and we have even, and this is probably only a mild exaggeration, saved the world once or twice as well (See: World War 2). We also laid out a set of principles that I think, if we lived up to them, and when we live up to those of them we sometimes live up to, it would and does make the world a better place. We don't always live up to our ideals and do the right thing, but I don't think I want to throw the baby out with the bathwater and say that destroys all the good ideals we have and the good that we have done.
I understand why a Native American or an African American might see things differently, though, and I am not trying to deny them their feelings. I get where they're going from. We've historically treated those groups very poorly and only recently begun to do better in the grand scheme of things, and need to get better still in the future.