It is very hard to sit by and watch, but I don't really know what to do. Do I try to console her, or just step back and let it go (which is killing me to do).
as difficult as it sounds yes, you have to step back been there, done that here's an example: how do you help an alcoholic? you can't. you can't help someone who doesn't want to be helped they have to figure it out for themselves that they have a problem and need help
talking about how hard it is will not do you any good don't just take a step back from w then take another step back from yourself and take a look at yourself what are you doing about you for yourself?
i know it's hard to hear but sometimes we all need a swift kick in the pants. i know i needed it.