During the teen years, it's normal to feel very self-conscious about one's attractiveness. Because our culture bombards us daily with unrealistic images of what is considered physical perfection, we're always coming up short. Despite your intelligence, you have become swept up in defining yourself and your worth by how beautiful you are. What's more is that you say that you not only believe that you don't measure up in the beauty department, but that you're disgusted when you look in the mirror. Since you look at yourself in the mirror excessively, you're spending a lot of time being unhappy with who you are.
I do not know what's caused you to become obsessed with physical perfection. I'm sure that we could begin with the fact that you have probably always had people telling you how beautiful you are, perhaps to the exclusion of commenting on your great personal characteristics. Maybe you have somehow bought into the notion that people will only find you worthwhile if you represent a physical ideal.
It's not normal insecurity for a healthy, developing teenager to feel this way. It's a serious emotional problem and it will get worse if you don't address it (this was the first step). Keeping this to yourself is the worst thing that you can do right now. It certainly would help if you could open up to your parents and other family members and friends. I strongly recommend that you see a therapist who has counseled many young women who have struggled with body-image problems. You need to summon up the courage to take action and to stop this obsessive thinking. Write me back and tell me what you've done to get started.