Others have mentioned some very good things. This is how I organized things before reading their answers. I hope it helps solidify your ideas:
First things first: If your girlfriend has at any time trusted Chrsit for her salvation, she is a Christian, PERIOD over and out.
Secondly, your pastor is quoting the Word when he says that Christians are not to marry non Christians--the principle of the unevenly yoked oxen.
Thirdly: If you and your girfriend practice first Jonh 1.9, confessing your sins directly to God every time you know you have sinned, you are both doing it the right way.
Fourthly: Yes, a fourth: If your Girlfriend is open to hearing new things from the bible, she is still humble before God and wanting to learn His word. If you stay together, you may become her teacher, both by example and in word.
So, my conclusion: I would stay with her, and one day if both of you come the the realization that God has put the both of you together, then you can look at marriage. What God joins together no person can put asunder. Of course the two of you can foul it up, but no one else can.
If you will love her as Christ loves the Church, and will die for her, then she may be your right woman. It again matters not if she attends church regularly or not at this point.
In case it matters I am Anglican (Church of England).
Charles "That Cheeky Lad"
P.S. All the best to both of you and...
A Wicked Happy Christmas to your and your girl, and everyone else who celebrates!
Peace and prosperity to all!