Beautiful is not only skin deep. A person has inner beauty. The smile, eyes lighting up with love/joy, etc. is just an outer expression of the inner self-love/self-esteem.

I would start with asking your fiance why he thinks that; what he sees that is beautiful and then take his word for it and look for it through the days yourself. I do that with my husband, ask him why he loves me, what he loves about me and what my strengths are and, because I trust his judgment, I just accept it's true but maybe I can't see it yet? So I start looking for it, knowing it's there and it's just my way of looking that might have troubles.