America has failed in it's lofty ideal many times, yes, but every step of the way shows an almost irrational desire to continue to correct course and hopefully, eventually, achieve that goal. The abolition of slavery, universal suffrage, reparations to Japanese internment camp prisoners... America loudly and proudly celebrated our first black president not too long ago. That says something about our principles. It's not a myth, it's a fact.
Not when it was founded, your argument was that it was exceptional since it was founded on those principles but it wasn't. USA gave rights to black people and women about the same time the rest of the western world did, they weren't ahead on this.
The US did not give rights to Blacks, etc. The US recognized those rights. This is the genius of the Declaration of Independence. This is exceptional.
The criticism that the Constitution didn't go all the way in the first go is a bit unreasonable. People forget how far it did go. For example, it rejected the notion of a nobility class. It explicitly rejected establishment of a state religion. We forget how exceptional this was at the time, we just take it for granted.