Girl said she didn't like me but...

So i'm 15 years old, and i've been in love with a girl since i was 11, and i finally decided to do something about. We started talking for the first time, when were in a study group together. I'm a really shy guy when it comes to girls. I can't be myself when i talk with girls (which i almost don't). So we talked, and it went pretty well. We had eye contact for about 3 seconds, without looking away (which was for me the best thing ever). I then asked her if she wanted to hang out sometime. I was too nervous to be myself. I did things that was so out of my way, because it was her. How i acted wasn't really who i was. She then told she doesn't like and she probably never will. It's been a month, and i can't get over her, because i feel like i could've won her over if i acted like i do with my friends. We had a connection when we talked for the first time, and that's probably why she went out with me. I can't stop thinking about how i could've done so much better. ALL the other boys in my class, talk to the girls. I'm the only one who doesn't. And when i finally had the courage to do so, i blew it. My friends tells me that i'm unsociable, though i talk all the guys with ease. Ofc i can talk to girls, if i need to (group work) but i don't act normal. I act so insecure and shy. So what should i do to become more confident, and do things i wanna do and not worry about how people look at me. And maybe win her over?