Congrats...again?
I didn't really get a chance to highlight this before, and I don't know if someone else did though I somehow doubt there was much support among the sea of "you're too young" comments.
My girlfriend was having some doubts about us (we're not getting married or anything) so we talked it out, and I basically told her we should maybe take it easy from each other for a while. We did, but the past few times we saw each other we tried to do things that reminded us why we got together in the first place; she's really an athletic/outdoors person, and so things as simple as going out with her and a bunch of our friends and playing football/frisbee made her extremely happy.
Try to do things like that if she's having doubts; think about what makes her happy. Not just flowers or jewelry (that works too) but what she likes doing with friends, and do that. If she likes going to movies, get a bunch of people to go to a movie. If she likes bbqing, call a few friends (make sure hers are there too!) and fire up the grill. Just having other people around in a social environment where she can be less concerned about you and her makes everything easier.