hey
are you feeling that you do not want the baby and that it is a mistake - if this is the case the you really need to talk to your girlfriend and quickly. It sounds from the posts earlier that you have a really good relationship, and that is not something you want to give up. Ok so a baby is on the way, its a big deal, but not something that isnt going to help your relationship, its will probably bring you closer.
If your worried about not been mature enough, then dont be. Your fiancee would probably have left you when she found out she was pregnant if she didnt think you could do as good job as a father.
All i will say is, Dont run out and not be there for your child growing up. It is one of the best times of your life