There's no law that allows killing of the unwilling; even a living will addressing assisted suicide or euthanization due to incapability assumes that you would still consent at the later date, but lack either physical or intellectual ability to communicate that. If you can clearly communicate that you've changed your mind, they have to respect that, even if that changed mind has reduced capability due to dementia.
Your best hope would be to go with assisted suicide while you still have enough faculties to make the decision and execute on your portions of the act.