I'm a 1st year PhD student at a University of California school, but it's not Berkeley or UCLA, so I have zero chance of getting a professorship anywhere, according to everyone I've ever spoken to. I'm hyper-aware of the outlook for TT professorships.
However, I've done some very cursory research on the high school teacher job market for History, and it appears that the situation is - well - not as bad. I really don't want to commit my life to this PhD anymore, knowing that I'll never find work. But I want all of your opinions first.
I'm going to go ahead and say this: if you're not sure, you're sure.
You should definitely quit unless the desire to complete is total, unwavering, and fundamentally essential to your being.
I did a little survey of people with PhDs here:
http://thomasthethinkengine.com/2014/06/12/should-you-do-a-phd/
and the vibe is: the work that goes into the PhD has to be reward in and of itself, because it his highly unlikely it will add any other value to your life.
EDIT: also beware sample bias. hanging around professional historians (even in this subreddit) who all have PhDs, can give you a sense of causation that does not exist in the real world, where most PhDs work in non-academic fields.