doubting my vagina's capabilities!

Okay, I know this is probably one of those common, crazy, FTM fears.. but lately I really am doubting my body's ability to give birth! I am 36 weeks and DH and I have taken the Bradley method classes so we are educated on and prepared to have a natural waterbirth. 
However, tonight for the first time, we did the perennial massage. He did it for me since I can't reach. 
It. Hurt. Like. HELL! 
I was almost in tears! Then I realized that this is nothing compared to a tiny human coming out of there! 
I have been sexually active since I was 16 and now, DH and I have sex pretty frequently, yet each time (and more so lately) it's very hard to get started because I'm so tight. 
I know the vagina is "like a rubber band" so being sexually active doesnt stretch it out or anything (which I loved as a teenager lol) but now I'm wishing it would stretch out just a little! Im so scared and paranoid that the baby literally just won't fit. I know some women have too small of a pelvis to do it, but is there a way to tell ahead of time? Like I said- I'm sure I'm being dramatic & every woman feels this way, but a little reassurance or advice would be really appreciated! I'm freaking out here!!