Can I make medical decisions for my mother as her healthcare POA

If your mother is not demented, she has the legal capacity to make her own decisions.  These choices may not seem rational to you but you cannot experience what she experiences or know what she knows, regardless of whether she articulates it.  It is hard to let someone go.  But letting people live their own lives, make their own choices, die their own deaths is an act of respect and love.  That said (and experienced), you might ask your mother's physicians whether depression could be influencing her decisions.