You are suffering for the loss of a dream, the denial of a wish, and that is difficult. You have your own ways to justify your feelings (conviction that your daughter would be better off with a sibling, for example). Your husband is arguing his case from a practical angle, but he may also have negative feelings about fathering and supporting another child that run just as deep for him. So the two of you have a clash of desires, and will do well to look at which of those desires are arising from real circumstances, and which are based on ideas, which are more available for change.
I'm now in my 60's and have known at least 4 dozen women who at one time or another desperately wished for another baby. I always thought little babies were the most wonderful thing in the world. But I stopped at one for good, practical reasons, and have never regretted it. Baby hunger? Yes, it has arisen, but a clear look at my life and my daughter's future always convinced me that one was enough in our circumstances. And as it turned out, she did great as an only child, as was so content with it, she's now raising a wonderful only son.
Most of the families I've known never regretted, ultimately, stopping with the one or two they had. But a few women who went ahead and had more did regret it, for any number of reasons. Of course they loved all their children, but some had children who turned out to be one too many for the emotional, financial, or energy resources of the parents, and that resulted in divorce when the strains became too great. Some additional children seriously affected the quality of life for existing children because of unanticipated medical or behavioral problems, or because a parent's health failed. Some divorced moms struggled to raise kids on their own. It has surprised me how often I've seen regret become a fixture in a family, and how often it breaks families.
On the other hand, in many, many conversations with my women friends, we have all reflected on the sheer joy of realizing our blessings, whether that was one child or two, or in a couple of cases, no children in spite of trying for years. Coming to terms with the limitations of life allows us to refocus and find a way to live fully and joyously within those boundaries. Please don't ever give your daughter a hint that she is not 'enough' to make you happy. Or that she can't / shouldn't be happy as an only child. That would be handing your unhappiness over to her to live out.
Onlies are happy, well adjusted children. Millions of American families now have only children. Those kids get to choose close friends, brothers and sisters as they move through life. Siblings are no guarantee of good companionship or emotional support, in childhood or in later years. I have two extremely needy sisters, in addition to a mom who's failing and needing growing amounts of my attention. I can't spread myself any thinner. I not only get no help from my sisters, I get demands that I'm should be there for them, and help them financially. Can't. My husband and I live on less combined than the average schoolteacher earns.
There's also the health of our beautiful planet to consider. Past generations should have been paying closer attention. They didn't, so parents today would do well to educate themselves. The population curve is no longer a gradual upward slope; it's a line running almost straight up. We'll soon be at a tipping point that will jeopardize the well-being of all the children we're bringing onto the Earth, and the very climate we depend upon will no longer be able to maintain itself.
Nature guarantees the future of the human species by making us hungre for more babies: they "feel" so desirable, in spite of the pain, discomforts, and sacrifices of birthing and raising them. That does not mean that humans can reasonably go on seeking to fulfill that hunger, any more than we can possibly eat all the food that would taste good. Baby hunger is hormonal, it's cultural, it's emotional, it's familial, and will probably be there no matter how many babies some of us have. But we don't have to dwell on it. We can live fully and be fulfilled in spite of it.
It is a choice. We who get past it focus on being thankful, and we let go of unrealistic or impractical dreams, and we live, right now in this amazing moment with the amazing child(ren) we do have. And if we're so lucky, we have a loving husband right there beside us.
I wish the best for you and your family. You'll do yourself and your loved ones a great service if you can open yourself to the possibility that your current wishes need not make you so miserable. Your misery is surely not adding to your family's happiness or your husband's willingness to commit to even more family. If you can focus on being happy, enjoying your child to the maximum, who knows how THAT might ultimately affect your husband's reservations? And even if he never comes around to 'more fatherhood,' you will still be much happier.