How should I break up with my girlfriend of 1 year?

We will be sophomores in high school and have been dating since april of 10'. She is always sad, doesnt liek to go out in groups, over emotional, bipolar. Im just tired of her. Her always texting me that shes sad and doesnt kno why. How she goes on facebook about every bad thing that happens between me and her. She just makes me mad now and im just anoyyed. I used to have so many friends and now I've just lost touch with them and rarely talk to them. Shes just been such a huge part of my life im scared to think of how id reunite and stregnthen my relationships with my friends.