Feel like my fiance isn't pretty enough?

I love her. I love her personality. And when it is just me and her i think she is very pretty. But when we go out i am kind of embarrassed. I see all these woman who are stunningly beautiful. And that is the kind of girl i want to marry. Like one of her friends is so pretty and i just find myself wishing that she looked more like her. Idk. I feel shallow. And i feel like i can't break it off with her just because of her looks. But i need someone that is better looking.