There's also an interesting by 'young people for young people' BBC documentary here.
Perhaps you might say that you and mum are thinking of watching it. You want to try and understand a bit better what she's going through so you can help. Perhaps she can watch it with you "in case you need...
That feeling of helplessness is horrible. Don't be afraid to seek counselling yourselves. You need a confidential place to unload and support from professionals too.