I know that someone suggested Canteen to you. Here is the link to their support page: https://www.canteen.org.au/home/how-we-help/counselling-and-individual-support/
It might help just to speak to someone with the professional expertise to get you over this time.
There is professional support available through BCNA as well: "BCNA Helpline team on 1800 500 258 between 9.00 am and 5.00 (Eastern Standard Time) Monday - Friday." They may be able to point you in the right direction.
Try to avoid Dr Google, now... Remember that a lot of the information on there is pretty alarmist and often quite out of date. Breast cancer treatment has come a long way in a short time and you don't know the particulars of your Mum's cancer.
Your Mum will be anxious about herself but just as anxious about you - that's not to make you feel guilty but to look after yourself as well. You haven't said if you have other support networks that both of you can tap into. Distraction during this time of waiting can be good - not necessarily anything major but maybe walks or visits to (understanding) friends, cooking or swimming - things like that that don't take any major commitment of thought or energy, but things that you and your Mum like to do.
Christmas can make it extra hard but you and your Mum will get through it. I can speak from experience on this one as I was diagnosed just before last Christmas and my kids (11, 12 & 15 at the time), while I know they were scared too, were a fantastic support to me.