Ugh, what a horrible situation.
To me, it sounds like this is a "last straw" thing for your girlfriend. It is worth looking back to see what has been bubbling in the background- and be honest about that.
If you want to save the relationship, you're going to have to eat humble pie. "I'll make sure my mum tells you if anything like this happens again" etc. It'll mean listening to her side and taking her seriously- don't dismiss her remarks, as she is clearly unhappy.
It's sensible to start thinking seriously about what happens if the relationship is over. You might want legal advice on property, child access etc.
Good luck!