You may desperately want a child, but agreeing with other posters, it's not at all necessary for your child to have a sibling, and they won't be missing out from it. I'm an only, I've always been very happy with it, and while I do have cousins, we're not at all close. I also have many friends who have one or more siblings, and I would say that close sibling relationships seem to be the exception far more than the rule. Most seem to see them just a couple of times a year, and some actively don't get on with them. And it's very common for just one sibling to be left with elder care, while the others don't contribute. So don't project all those hypothetical reasons onto your child, please own that it's purely your own desire, and that's fine.
You know what I did find damaging though? I was an accident baby, and although I was definitely loved, and they chose to keep me and stay together, I knew I was an accident, and that did hurt. Your husband is absolutely right to say no if he doesn't want another one - children deserve to have two parents who are fully into and committed to the idea of having the baby.
At 7, the gap is already pretty much too big for them to enjoy each other as siblings anyway. Looking into the future, a 2 and 10 year old, or 6 and 14 year old won't have much in common anyway, or much desire to play together.
Appreciate it's very difficult, and sending you very much sympathy, but maybe your energies/therapy would be better spent reframing your thoughts into focusing on having a wonderful relationship with your DD and pouring love into the relationships you do have, rather than yearning for something that in reality might be much worse for your existing DD, worse for your husband, and carries no guarantees for you that it will turn out as you wish.