How can we ever be "done" having children?

KP
Hi everyone, I'm 32 and pregnant with my third son. My first two are absolutely gorgeous little guys. I'm already madly in love with this one. My husband is 40 and he is SO ready to be done having children (has three additional kids from his first marriage). I thought for sure that I would have this final, contented feeling that I was ready to be past the baby stage, but I don't. Heck, I'm 31 weeks pregnant and coming off a night of where my current youngest barely slept. And when I was cuddling him in the middle of the night, I just felt this panicky feeling: "how many more nights will I be able to do this?"
How does one accept the fact that the last one IS the last one?? (No, this doesn't have to do with boys vs. girls; I would gladly keep having children even if they were all boys. I never have had a gender preference.)