TishaBuy has some valid points.

Your mom is more than merely Depressed. Her family doctor is a good start.

Has your mom expressed why she is adverse to seeing someone about her health? Perhaps she thinks she is doing family a favour, perhaps she is worried about what people will think.

I realise my mother was likely bipolar. It would certainly explain a lot. If she were alive today I would somehow find the strength to confront her. I would advise her of how her illness affected me. I would advise her she was NOT doing me a favour. I would advise her I was having the intervention because I cared. My conversation would proably be something along the line of, "Can we agree something is wrong? Can we agree this isn't working?" "Who will make the appointment? You or me?" I honestly do think that sometimes we need to reverse the roles of parent and child. I would also ask her what she would like me to do.