How do I get power-of-attorney over my mom if she doesn't want to cooperate?

In order to put a Durable Power of Attorney in effect, a Dr. Needs to deem her incompetent to make her own decisions. A court can too, but its more complicated.

Depending on the degree of Mom's mental issue - it may be too late for a POA.
Look on this page upper right under "Money and Legal" then POA & Guardianship. Your question is covered in detail there. Good luck.