OP, I think you can be grateful you don’t have any more children.
The man was going to turn out to be a bastard whether you had two or three children, and if it’s only the baby you’re morning then clearly the marriage wasn’t in that great a state anyway and it was probably best not to bring another baby into it.
Far better that you find out before you had another baby than while you were pregnant or had a newborn to contend with.
I had a similar situation wrt having more children. Not exactly the same but I suffered secondary infertility and we ttc another baby for six years. I reached the decision after so long that the age gap was too big, that I didn’t want to spend more time trying to have a baby I couldn’t have, and that I thought it would be better for me to stop and go back to work instead (I was a SAHM). So we called a halt. He didn’t want to, I did.
Anyway just over a year later the marriage ended for various reasons. And looking back I think about how I could have ended up a single parent to two children instead of one.
And being a single parent is hard. There’s no point dressing that up. Especially with tots because you’re doing everything for them, and depending on the level of involvement your ex has with them it can turn out to be relentless.
you have two children. That’s a lot to be thankful for.
There’s no knowing that you would even have had a third baby. Nothing in life is guaranteed.
And there’s not a chance I would go down the route of sperm donation just for the sake of a having a baby, a baby you will 100% be a single parent to 24/7, a baby which could turn out to have disabilities which could make it extra reliant on you.
You could meet someone and have another baby, if it’s meant to be then it will be. But sometimes it just isn’t, and you have to focus on what you have.