I have authority w/Gen Durable POA & Health Care. Enforced upon signing a few yrs ago. Question-how to invoke so she can't override me now?

You need to have this enforced with a good neuro-psyc exam. If she is adjudged competent then you cannot use the POA. An individual is allowed to make his or her own decisions about medications and about everything else UNTIL adjudged incompetent under the law. If you think you need more, then you would need guardianship. And guardianship isn't easy to get. Costly and your Mother would be appointed an attorney to represent her, and if she is even MILDLY competent she would win. The judges in our country are VERY loathe to take the rights away from a citizen.
What decisions are you making that she is trying to override?

I suggest you take her to a neurologist and have labs and cognitive testing done. Then have him/her write a letter saying Mom is no longer competent to make informed decisions.