Let me just be brutally honest with you here.
You may be feeling conflicted because you don't want to somehow abandon a girl that is with child, but you have to consider this: you are both adults, you have to make the decision that is best for you, in this sitituation, it would not be selfish. You are at the beginning of your life, unless you feel that you are ready, do not take this on.
Did she get pregnant with the baby while you were together?
I understand wavering faithfulness, but, how are you going to feel when that baby is born?
I am not trying to be mean, I just feel like in this situation you need to consider yourself, since the child is not naturally your responsibility, because it is not your flesh and blood.