It is my understanding that he was bitter, until Lincoln challenged him to read the Constitution and Bill of Rights for himself and not just rely upon what he was told about it.
This was his speech in 1852.
In 1852, he delivered another of his more famous speeches, one that later came to be called “What to a slave is the 4th of July?”
In one section of the speech, Douglass noted, “What, to the American slave, is your 4th of July? I answer: a day that reveals to him, more than all other days in the year, the gross injustice and cruelty to which he is the constant victim. To him, your celebration is a sham; your boasted liberty, an unholy license; your national greatness, swelling vanity; your sounds of rejoicing are empty and heartless; your denunciations of tyrants, brass fronted impudence; your shouts of liberty and equality, hollow mockery; your prayers and hymns, your sermons and thanksgivings, with all your religious parade, and solemnity, are, to him, mere bombast, fraud, deception, impiety, and hypocrisy—a thin veil to cover up crimes which would disgrace a nation of savages.”
That sounds bitter to me. But it changed when he was challenged by Lincoln to read the Constitution and Bill of Rights. When he did that, that’s when he advocated for America to be for all people.
No, in the same speech, he explains how the Decleration and the Constitution carry Freedom in them, he fervently praised the Founders, Garrison was a founder of today’s leftists.
Bear in mind the preface of the speech: “What, to the American slave, is your 4th of July?" Speaking the truth is not being bitter!