What are the legal issues surrounding a Durable Power of Attorney (DPOA) vs. a conservatorship?

If I already have a DPOA, is a conservatorship even necessary? Mom has Alzheimer's/dementia memory problems which are affecting her mental state. She agreed to sign a DPOA with me as her agent back in March, and I assisted her in setting up and monitoring her bills.

Due to circumstances that are explained in more detail in my profile, I believe that my mom needs to be under a doctor's care. I cannot get her to see a doctor (for any reason) - but I feel she is no longer competent to manage her own finances - I want to step in and help, but by doing so, I know she will be convinced that I am "stealing" her money. I've set it up twice now, and both times, she's gotten paranoid and closed her billpay account. Now, her creditors are calling me with her bills two months past-due.

My question is, with a Durable Power of Attorney currently in force, can I step in and take over her bills even if she doesn't want me to? How do I prove that she's incapable of making this decision anymore - should I go to court, to a doctor, or what?

~FyreFly