Here's a question.
If your fiance would leave you because of that, is that really the kind of guy you would want to spend the rest of your life with? He loves you because of who you are. If he didn't like it I'm sure he would've said something when you started losing weight. Congratulations on that by the way

As for the question of if breasts are important, I personally think they're useless (I don't particularly like and never want to have children, so to me they are). People think I'm much younger than I am, and not just because of my younger looking face lol
But I don't mind, I'd prefer guys look at my face when I'm talking to them and when I date people I'll know that the person likes me for who I am. Plus I'm a pretty athletic person (martial arts mostly) and I don't really know but I would think having large breasts could eh, get in the way a bit lol