Like RA B, I think it has to do with the knowledge that it has an end.
I actually was sick like you describe for the entirety of the pregnancy. Until I went into labor (which was past the 40 week due date). It was miserable. The doctor said that it isn't a guarantee that it would happen with future pregnancies... still, we waited 2 years. My second pregnancy was worse than the first... sick every single day until day of delivery... only she came a week before due date. I had to go on Zofran to be functional during the the first trimester and part of the second.
After that, we were done. It wasn't a matter of not wanting to be sick, but rather a matter of there being no way to manage our day-to-day lives if I were as sick with a third pregnancy as I was with either of the first. We had to send our first born to stay with relatives for part of the 2nd pregnancy (before I went on Zofran) because I could not take care of him. I couldn't function. You can't be like that and have a toddler and a 5 year old running loose in the house.
But during the darkest days, knowing that it would go away after the baby arrived just made delivery day that much sweeter.
The question it seems is what do YOU want? You haven't said whether you are willing to endure for one more. That's up to you. Your choice. If you can't face it, then you can't. No shame in that. Adopt if you would still like more kids and can't face the misery of round the clock nausea again.
For some people it gets worse with each pregnancy. So know that before you decide. There are no guarantees either way--you might not be sick at all, or it could be worse... pregnancies are not all the same.
Only you can decide. What is tolerable for someone else is just what is tolerable for someone else. Not what YOU should do or not do.
How does your husband feel about adoption if you don't want to endure another pregnancy? You didn't mention that. Something to add to the mix when you are deciding whether you want another child, when that might mean sickness or adoption to attain another child.