I say the same as everyone else - show her she's beautiful, don't just tell her. Telling her could sound like you're just saying that because you know you should. But if you pay special attention to her, surprise her with flowers or just a hug while she's cooking... I mean the little things. I know my boyfriend thinks I'm beautiful, and I love when he tells me that. But I think it sinks in a lot more when I catch him admiring me while I'm getting dressed or when he just stares at me and smiles...that sounds creepy, lol! but you know what I mean... when you look over and catch someone just basking in how much they love you, it gets you a little giddy! Try and notice when she's done her hair, bought new perfume or is wearing her makeup a different way. She's doing those things to feel pretty, and having you notice will let her know that it worked
