stop being lonely

When I was in the kindergasrten, I actually had only my sister with me. I also started school with her just to avoid being alone in class. She always was more sociable and got faster new friends, just because she was prettier than me (yes, you heard me). Later, in the third grade, I had friends on my own and everything was okay till the sixth grade. Then we moved and had to change the school and there were totally new people. My sister went into another school. I found “friends”, but they actually never really were. I was just hanging out with them in the breaks, after school I was all alone. A year later I kind of got friends with other people in my class and then my “friends” changed quite often, but till now I never meet them after school. Just now and then I get invited somewhere. And I feel really sad when I see such big groups with those “cool” people and they seem so happy having so much friends and always do something with them and so on. Also it kind of hurts when I see them in the floors and they see me going to another room, all alone, and sitting there doing already some of my homework. And they can spend their break talk to friends.
The biggest problem is, that it just seems, that I simply not extrovert and cannot talk to people without being kind of… dull. It is just my character. I know about my problems, but it is just the way I am and it is hard to change so much, just to be more integrated. But otherwise I will never have friends or other social contacts…