How to get my girlfriend to be more comfortable around me?

I'm 15, and my girlfriend is 14, and we've been together for a little over 2 months now. I'm really liking her a lot. =] The only thing is, it seems like we've been together for a long time, but she's really not an affectionate person at all. She'll hold my hand, and give me hugs when we say goodbye, but... I don't know. Just not real cuddly, and doesn't like to be too close when we're sitting down and stuff. Like, if I move too close to her, she'll kinda move away, so we're always, like, a little more than half a foot away from each other while sitting down and stuff. I don't want to force her to do things she doesn't want to, make her uncomfortable, or change her. But any suggestions on how to kinda get her warmed up to me more? I just like her so much, and I'd really like to be able to cuddle with her a bit more. Thanks in advance for any answers.