“…and I now resolved that, however long I might remain a slave in form, the day had passed forever when I could be a slave in fact.”
Douglass is saying that although he might seem like a slave and be a slave to others, he would never see himself as one again. He did not consider himself property any longer, but as a human being. This is after his fight with Mr. Covey. Earlier, Douglass says that Covey managed to completely destroy every aspect of himself. He said he was broken in body, mind, and spirit, transformed into an animal. But once he won the fight with Covey, he was renewed.
We see this distinction throughout the rest of the Narrative. For example, when he says “I will give Mr. Freeland the credit of being the best master I ever had, till I became my own master.” Douglass is no longer content with having a “good” or “kind” master anymore, because he has already tenaciously decided that he is no longer a slave.
Additionally, although Douglass gets caught before even trying to escape the first time, he does not back down and accept it. He continues working as a slave physically, but mentally has already escaped. Then, he finally makes it to New York where he becomes to others what he was already to himself: a free man.