Welcome! You sound like you have a good understanding of what narcs are like, and how they operate. Most of us have done the back and forth thing, so try not to be too hard on yourself. And you probably are aware that, if he is truly a narcissist, there is no changing the fact.
Please do a search on this site about divorcing a narc, there is a LOT of valuable information posted. The support you will find here is priceless!
Looking back I should have left my N when I was pregnant. That was the first time he was obviously, unmistakingly abusive to me. It is so hard to leave when you are in that kind of situation. When my baby was 2 weeks old we got into a bad argument and he threatened to take him away. I said "how could you take a 2 week old baby away from him mother". He said "I know how to make baby formula, and he doesn't need someone like you and your mom raising him anyway. He is such a pig. It is hard to leave early on but It is almost always a waste of time to stay. I heard Oprah Winfrey say one time that when someone shows you who they really are. Believe them.
Paloma
Paloma