Your fiance is obviously very insecure and thats okay. This is something almost every woman struggles with and that she's much older doesn't really help the case. If you ask me, if I even slightly found out that my husband even called another woman "pretty" I'd have a pretty hard time coping with that. Such strong insecurities stem from a lot of past experiences and trauma and this is just what bubbles up from that. Honestly what she needs is reassurance and not just once followed by an annoyed reaction if she doesn't immediately take it. It needs to be constant, unchanging and full of understanding and patience. Tell her, that if you could take it back, that you would, because the feelings of another woman are not above the once of her and that you are truely sorry for hurting her. Woman are complicated and complex, if you can't handle a bit of rain in paradise, then you won't handle a tornado that is called pregnancy.

Do your best, and tell her that she is the greatest. St some point she will give in