Just don't worry! If there is no cause for concern and your dr says that you and baby are healthy then you've got nothing to stress about mamma! Congrats
I'm 19 and I felt the same way, I felt like my body wouldn't be able to carry this baby, I'm 38 weeks and 6 days now, and my baby is perfectly healthy and strong. That fear is common for most first time moms specially if really young. There's really nothing I can say you should be worried about, don't stress yourself out too much with worrying. Just take care of yourself properly, eat well, take precautions, follow doctor's instructions. By doing those things you're helping yourself and the baby. Don't worry too much