One child or more, does not make a family.
More children, does NOT mean, everything will be close and cozy and hunky dory.
It does not mean, the siblings will get along.
It does not mean, that having a girl or the opposite gender... will make it a family.
It does not mean, that having a girl, will make the family close.
It does not mean, that having a girl, she will be close to you and remain, close to you.
It does not mean, that having a boy, they will leave you and forget you and be closer to the in-laws.
It means, there is no guarantee, that having a 2nd child of any gender, will make things a certain way or not, or make it a "Hallmark card" feeling of a family.
The thing is, there is NO guarantee, of how things will be, once you have more kids. Of either gender.
Siblings, do not always get along. No matter what age they are.
No matter what gender they are.
And, each child, is different. Even if they are raised the same.
Gender, does NOT determine, the atmosphere of the family or the closeness of it or not.
Age difference between siblings, also does NOT, determine if siblings will get along or not.
You do not seem, to want another child.
And your marriage, does not seem to be able to handle another child.
Whether it is having one child or more... having a child or children... IS a handful. And it is not easy.
So you have to determine, if you or your Husband, can handle... another child in your family.
Because, no child should be "resented" by the parents or other sibling. It will affect them, negatively.
NO child, should feel it is their fault, that the parents do not get along.
And a child should not be looked at, as being something to make a family feel closer.
You and your Husband, do not seem, to need or want another child.
You both have marital problems, since you had your current child.
It will probably get worse, if you have more kids.
Kids do not, "fix" a marriage.