I am the parent of an only, and he will be staying an only child. We decided not to have more children because we both have serious physical and emotional health challenges. And our son needs living and functional parents more than he needs siblings.
We have come to peace with this. Yes, there are still times I worry about things he might miss out on from not having siblings. But he is doing well. Only children can be well-adjusted, intelligent, friendly, kind, and accomplished as well as those with siblings.
Only children often spend much of their time with parents, and thus develop adult language and reasoning skills fast. They get more one-on-one time and attention, and statistics show they tend to do better in school(in general-of course each kid is unique).
I've heard over and over that "he won't learn to share." In fact, MANY people have commented that our son is better at sharing than most kids, and has been from a very young age. I attribute this to his being an "only." He's never had to feel the very real fear that there is "not enough to go around" that comes with sibling rivalry. So he has a natural trust that there will always be enough. He can give to others without worrying that it takes away from him. He knows he is the center of our world and always will be, so he doesn't need to feel jealous. As a result, he can be more generous with others. He is very kind and has never learned to use the put-downs I hear many kids use with their siblings.
Parents of only children are less likely to be exhausted, and more likely to have energy, time, and sufficient money. This can be a blessing for you, your husband, and your child, if you will let it. You can really enjoy each other, take trips, give your child more opportunitites...
Because it is easier to give so much, we've made a conscious effort to teach our son self-sacrifice. Kids with siblings get this lessons automatically (though like I've said, they don't always take to it without rivalry). It's easier to let the world revolve around your only than when your attention is divided. We require our son to give things away, be involved in service to others and chores, etc.
I would absolutely NOT pressure your husband to have more children. I know you are sad about this. And I get it, as my husband was the same way. I was willing to sacrifice our health to try for another, but it would NOT have been a good thing. Now that my health is even worse I'm so glad I listened to my husband about this. Our son would be in real trouble if we'd had more kids. I barely get by as it is with our health challenges, and he needs parents who aren't overwhelmed and sick all the time.
Your daughter needs her parents to have a stable marriage more than she needs to have a sibling. That will be the best gift you can give her. Think about it this way: If you or your husband had a condition that would make having more kids physically impossible or life threatening, you would probably be able to let go of the idea. You wouldn't ask your husband to sacrifice your life or his and give up the great blessings you have right now.
Tell yourself that your husband's refusal to have more kids is his way of telling you he is (emotionally) unable to have more kids. He is telling you he has limited resources, and does not want to get overwhelmed and lose the peace you both have right now. Emotional limitations to having more kids are real, just as are physical ones. If you tell yourself your husband "can't have more kids" and maintain his equilibrium (which is what he's trying to tell you), you can let go of the anger that can come if you tell yourself you "can't have more kids" because "my husband won't let me."
Try to focus on the wonderful blessings you do have in your life. Look at the positive things which can come from having only one child. And know that you are not alone. MANY people are in the same boat. But you and your family can be ok. Just work to make your little family the strongest and best little family it can be, even if it's different from other families.
I would agree with what one of the other writers wrote about grieving and letting it go. What will be hard on your daughter if she's an only is if she knows you are disappointed about that. If you show her you are happy with the way your family is, she will be too (and won't know any different from her own experience).
Please know in sending this post I do not intend to judge anyone's decisions. There are many types of happy and successful families, and family size is a very personal decision. No one else can say what is right for you. You have my warmest thoughts and lots of hugs. :o)