Ok, so supposing she does dump him and splits up her family, then what.
She's a single mother aged 38, and then she sees her daughter, 50% of the time, because she'll be with her dad. The other 50%.
There'll be less money because the house will have to be sold. That will all take time by which time the op will be pushing 40
So then she jumps straight into dating and tries to find a man who's serious about marriage commitment, and having a child pretty much immediately and also new partner immediately becomes a step dad. OP may also become a step mother herself taking on somebody else's children and that wont be easy.
Her DD loses her stable home where she is the only child with 2 parents both there., just because mum wanted another kid.
That's if OP even finds somebody else to have a child with. She may end up permanently a single mother with one child who she only sees half the time.
How likely is it that dumping her husband will work out? I'm afraid it's not as simple as you put it, just dump him and find somebody else. There's also difference between lying and changing your mind. Hes changed his mind and doesnt want another child.
You see what most people fail to do is realise nothing is perfect and truly accept it and be at peace with it.
I’m not saying you should stay in a comfortable marriage if another child with someone else is what you'd rather have.
But that if you were to want to leave them what would your alternative reality truly be? Would it truly be better, would it truly make you happier or are you just HOPING it’d be better?
That is your issue. Are really hoping it’d be better versus being confident it’d be better.
Hopefully you can evaluate your alternatives better before you throw yourself in to a worse relationship predicament than your current.