I was once in a place like yourself. I came to learn that physical attraction can help start the relationship, but almost everything else is what keeps it going. My fiancee is a heavy girl, and initially it is part of what attracted me to her. She hasn't lost much weight since I met her, but we're living healthier than she was. Unfortunately, she is diabetic, but it's not to the point where she has to eat a very specific way or take medication regularly. She wants to lose the weight and I've told her if that's what she wants, she should go for it - after all, she's beautiful and will be whether she's thick or thin. I would prefer her healthy than heavy. I've seen pictures of her thin and she's just as beautiful - so while it was (and still is) a perk for me, she doesn't really want to be heavy, but I guarantee you she's much more confident in her appearance now than when we met, and she definitely feels sexy around me.

Make her feel beautiful, don't try to convince her that she is - it works SO much better and you'll see her confidence soar. When she says she wants to lose weight, let her know that it should be for herself and nobody else, you like (or love) her just as she is and she needn't change any of that for you. If you would still be happy with her if she's thin, or thinner (and hopefully you would be), let her know that too. But don't be pushy about it argumentatively. Try to show it physically. SHOW her how beautiful you think she is, don't just tell her. Hope that helps!