I am not 100% sure your fiance is Narcissist, but either way, being an abuse survivor myself, I do know this fits the classic cyclic pattern of abuse in general.

Honeymoon Stage - everything seems normal, even extra special at times. You start to believe it can always be this way and consider yourself lucky.

Escalation Stage - little things start upsetting the abuser, and it is always made to be your fault. "If you would just do this then I wouldn't do that" etc.. only no matter how much you follow requests, it's never good enough, there are always more, and the anger coming from the abuser gets worse and worse each time

Explosive Stage - The temper is most violent, and you don't even necessarily have to do anything to trigger it but nonetheless, blame is always yours to bear according to the abuser.

After a time, the abuser becomes apologetic, and then the cycle begins again.

These stages are done over a long enough period of time the shift between them is not obvious, but the pattern, after being used on a person for a long enough period of time - becomes a form of brainwashing. It has been shown to be similar to the same kind of thing POWs were put through. It is not your fault it happened. It is not your fault you stayed as long as you have - but now that you have broken out of the brainwashing enough to realize it for what it is, it is time to start plans to leave.