So far as I know, and this I could be wrong depending on where you live,
children get to decide these things when they are 18 years old.
You have been separated 12 years, and not as yet divorced? I think it might be time, to um, well, you know, flush the toilet on this issue or pull up your pants and continued to be married.
If I were you, I'd talk to my lawyer, as this isn't really something strangers on the internet can do much more than point you in a hopeful direction.
Again, if I were you, I'd take this one step at a time. I have absolutely no idea what your ex's circumstances are. Should your daughter come to live with you, then your ex may have to pay child support. At worst, I don't see you would be paying your ex child support if you have the child.
But I think we are getting ahead of ourselves on this.
Others here likely have better thoughts than I do about this.