How do I care for my mother when she refuses help?

I'm at my wits end and seriously need some help.  For the past 8 months I have been caring for my mother who has COPD.  In Dec. she came very close to dying because she refused medical help.  Only after she was unable to make medical decisions was I able to make the life saving calls.  I've since had to sell her home and move her to where I live.  She lived with my family for 2 1/2 months and then insisted on living on her own.  In the month she has been on her own she has spent 1 week in the hospital due to not taking care of herself.  During her last hospital stay I discovered a pack of cigarettes (all but one smoked) while putting away groceries for her return.  I'm spent.  I feel I've done everything I can do to help her.  My friends say it is time to take over and put her in assisted living using my power of attorney.  I've called my Dr. and talked with the case worker when she was in the hospital but I've gotten no help.  I am her only child, she has no husband.  I get support from my husband and two teen daughters.  My mother has and older brother and younger sister who live a long way away.  Honestly, my mother has alienated anyone who has ever tried to care for her all her life.  I left home at 17 and now me and my family are all she has.