What to do if I don't find my gf attractive anymore?

My girlfriend has gained a lot of weight since the past year. She constantly asks me if she looks good, is she fat and I always lie because I know that she struggles with her appearance very much and I don't want to hurt her feelings. I tried convincing her to diet or work out with me and said that I wanted to get more healthy but it didn't work. She REALLY doesn't want to diet, because she has already been on a serious diet once and she says that it was too hard. She had to use her own money and had to cook for herself but she doesn't have the time to do that (we're in high school, she still lives with her family). Also her family eats very unhealthy. Right now two of our friends said that they are starting a diet and I said that I want to join them because I thought that maybe she will feel weird being the only one not dieting. But it didn't work. She asked me if I think that she should be on a diet and I said that it depeneds on her if she does want to. She asked me if she is fat and I lied and said that she isn't so right now I know that she won't diet with us. It wouldnt be so hard if she stopped constantly asking me if I think that she's beautiful and sexy. She said that I dont look at her the same way that she does at me, she says that when I'm naked she cant take her eyes of me and that I dont even seem interested when she is naked. I didnt even notice that, I cant fake being attracted to someone if I'm not. Breaking up with her is not an option. I love her very much but I have no idea what to do. Please help me.