I have the same problem... not to the point of obsession, but I don't feel sexy like I used to, I guess because I am 46 now and not 18 and have had 4 kids and a few health issues that would have probably done most people in.
At the end of the day, he can think I am sexy all he wants to, I don't feel sexy at all.. and I know he means it from the very pit of his being, but it doesn't help me to think I am still sexy.
Women go thru this, especially if they are aging or have had kids and don't fly back into shape like they were before. And to be straight honest.... it isn't going to happen in most all cases.
Just keep complimenting her and letting her know she is what matters to you, that no one else can love and cherish her like you can, she is beautiful, all of this line of thinking. Don't YOU obsess about her being obsessed, just keep telling her how much you love and care about her. That is all you can do. I honestly don't think there is one thing YOU can personally do to make her feel better about herself, she has to feel that way about herself.
Women are odd creatures.. you can think we're the hottest thing on 2 legs, but if WE don't think so, little can be done until we decide we are who we are here and now, we aren't 18 anymore. Until she realizes this..... just do your best to make her the center of your world, and support/comfort her.....
