What do you call a person who does not stand up for themselves? I'm looking for a word to describe how a slave thought of his kind back before the Civil War started. The word have to be a synonym to weak. It also has to describe sameness. This word also has to mean fitting in by force, but is OK with it.
There are a few different options covering various aspects of your request. "Passive" literally means not taking action, while "submissive" or "subservient" would mean acting the way a slave 'should', but they don't carry any connotations about being 'okay with it' - you can act subservient but still resent your position.
A slave might be described as "knowing their place" if they are seen to accept their position. "Stockholm Syndrome" is a general term for the phenomenon of someone identifying with their captor. Terms like "Uncle Tom" or "house negro" have been used to describe slaves complicit in their own slavery or the slavery of others, though these are fairly inflammatory and shouldn't be used casually.
It's important to note, though, that most slaves would not have thought of themselves in this way - the idea of the happy slave is largely a myth, at least within the American chattel system, and slaves resisted in lots of ways besides open rebellion.