I've had the pleasure of being the first guy for a couple of virgins...first one was a snap...broke through fairly easily. The second was much harder and went through the same thing you mentioned. It took a *lot* of trying, over several *months*. It finally happened and now everything's peachy, after 5 years
Moral of the story: every girl is different. Some girls will have a much thicker hymen and will take a lot longer. You *don't* want to rush it because you could really damage her.
She may also be very nervous, and is clamping up. She may think you're too big and is clamping up because of nerves. Maybe get a 4"x3/4" dildo first. Once she can take that, work up to 1 3/4", and then yourself. The smaller size, and letting her handle the dildo in by herself may help...