Am i just being really selfish?!

I feel like the most selfish person in the world. why? because i'm not happy. I'm an 18 christian girl. I was raised in church and a very christain family. everyone is always telling me to be happy and why i should be happy. they always say, you have the perfect life, the perfect parents, the perfect opportunities. my parents expect me to always be happy because i'm a christian. If they knew how unhappy i was they would think i was terrible and don't love God anymore. I do still love God and i'm trying to get closer to Him again. But i feel like everyone would judge me if they saw the parts of me i hide. I put on a happy face. I smile and laugh. Or mostly just stay in my room so they can't see how i'm feeling. But really, for over a year now, i hate myself. I feel like there is something wrong with me, but everyone always tells me why there shouldn't be. I feel like i have to be perfect to fit up to everyone's standards. But i miss those standards by a long shot! I feel like a failure in everything. I can never seem to get stuff done and then it piles up till i'm overwhelmed. I keep trying to lose weight but then pig out. I hate the way i look. all my friends tell me i'm pretty and not fat. But i feel huge and ugly. Sometimes i look at myself and cry and feel like screaming. I also hate my personality. I always feel awkward and stupid. My moods are all over the place. I do have good times where i "feel" happy. But they don't last and then sometimes i feel even worse. I also worry about everything. Sometimes because of my self-hatred, mood swings, and anxiety, i cut myself. I know it's unhealthy. I want to stop, but i can't bring myself to talk to a doctor. I don't know why i have all these issues. I have everything anyone could want. So many people have so much less. I feel so selfish and this only makes me hate myself even more. Am i a terrible person? What's wrong with me?? :(