I can't really say anything new, so I agree with what most of the previous comments have stated.
1. If your friend is just/only a friend, then you should have no problems keeping her as such.
2. Your GF is younger then you and that may cause her a certain amount of paranoia/insecurities. Live with them, but only while continually trying to ease/erase them. Constantly remind your GF just how much you care for her and only her.
3. Good friends are some of the hardest things to keep hold of. Don't loose one of the things that not all people can say that they have. A close relationship with an old friend.
Good luck with your situation, and remember to keep patient. Anything can be accomplished with time, even getting over relationship fears.
