I think you need to make her feel beautiful, rather than telling her all the time. Save compliments for those special moments, and when you do compliment her dont always focus on her looks. A big part of self esteem etc is about her worth as a person, and that you love her for her mind, her sense of humour etc, rather than the way she looks and compares to other girls. Take her to a spa day or shopping, treat her to something nice and take her out for a lovely meal followed by a romantic walk. Bring the romance and mystery she always wanted growing up, it will hopefully make her feel special and loved. Sometimes when she tells you she feels bad about herself, she may just need a cuddle and someone to let it out to, that can be more important than constantly telling her the opposite of what she's saying.