Here is the site of the Australian Samaritans. Please keep their number close and call them any time you feel distraught - they also do email contact if you don't want to speak. It's for anyone in emotional turmoil, and that's where you are right now. Your local MH services might also have some sort of crisis line so have a look if you can at least get in touch with them if not go in at the moment.
For the Christmas period, it sounds like to you need to get a plan in place so you feel less overwhelmed. The way you describe it, Christmas is like this avalanche that's rushing up to smash you and there's nothing you can do about it - if you can take control by making decisions about how to get through it, the sense of panic might be less.
Speaking for myself, I would probably not feel better avoiding family if it meant spending a lot of time alone. Will you have people around you if you decide not to do the drive?
If you do go, would it help to call your mum in advance and tell her that you're not feeling well and are going to need a lot of rest time during the holiday? She might not understand but at least you will have stated your situation upfront and will have a bit more weight behind you when you need to duck out of social events. You could ask her for the calendar of everything that's planned and you could decide ahead of time which you want to go to, which are definite nos and which you'll do if you feel up to it - maybe think about any measures you can take to make things doable, like taking your own car on visits so you can leave early if necessary.
What sort of thing do you think would be healing and restful for you over the next week? Make a resolution to do a 20min walk per day, have a healthy breakfast or go to bed by a certain time each night - whatever you can do to make a structure around you to take some of the strain. Maybe take a favourite book or music that will help you make a safety bubble you can go in when you feel stressed. If there are people you're looking forward to seeing, try to make plans to spend time with them one-on-one so you can enjoy their company without the crowd. Is there any part of Christmas that you enjoy, like a carol concert or preparing a particular dish? Try to make sure you get to do it so there's something less stressful to take your mind off the pressure.
Is there any way you can break the journey there and back, or even fly instead? I know many Aussies consider a 5hr drive a leisure activity, but it sounds lonely and exhausting to me and if you can get out of it that might help with the whole feeling of dread.
I hope Christmas turns out better than you expect, and the New Year is better than this one.