Call child support services and inquire as to whether or not they've checked in with your ex to see if the children are up to date with their pediatrician and/or eye doctor.
My ex does the same. I take them in for everything, and even though he's supposed to be paying 1/2 of the out of pocket costs he never does. I've seen the teeth nearly rot out of my son's head because of his neglect. Yet he gets my tax refunds, stimulus checks and child support.
Because she's a bad parent and she's allowing her child to take out her anger on you.
I wish I had better answers for you. I usually end up just paying the medical bills myself, which has ruined my credit and put me in a hole financially that will take me months if not years to dig out from. My only consolation is that I know I'm doing the right thing.
How about passive discipline? For instance, when your daughter said that you simply tell her that it's not for her to worry about and you won't discuss it with her any further.
BTW, there are a lot of medications for Bipolar Disorder. Are you on anything? If what you're on isn't really working you might consider something else.