If it's any consolation, I was in the lift on my way to the delivery suite, after having been in labour for 58 hours, and suddenly realised I didn't want a baby after all, I was too young, and definitely not ready. I even told the midwife I'd changed my mind!
That said, once he was born, I turned from timid to tigress, as most parents do, and 30 years on, I look back and see that I really wasn't prepared because nothing can ever truly prepare you for your experience, as even if it's a textbook pregnancy, birth and upbringing, it is still uniquely yours.
I went on to have three more, the second one just 14 months after my first. Whether they admit it or not, most first time mums (and second-timers, third etc.) all feel a sense of...maybe not doom as such, more apprehension? The sense of going into the unknown.
for you, and all the very best with the birth and motherhood, you're in for an amazing new life 