Weight issues

All my life my grandparents have told me I'm fat. So naturally I've always had body issues. I hate my body. Once someone tells you something over and over you start to believe that. So I went downstairs and I got a piece of my dads birthday cake just now. And my grandpa of course calls me fat. LIKE first of all one slice of cake isn't going to make me overweight. Second, I'm allowed to treat myself every oncein a while with something. I work out almost everyday and I'm currently on a diet with my best friend. Trying to motivate eachother. This just pushed me over the edge because I just hate the way I look. I can't help it. I've tried so many things. Sorry I just had to rant. I'm trying so hard to love myself. Any tips?