Do you want to stay in this relationship as it currently is? Has your girlfriend consistently ignored your desire for input, your wishes and needs? If so, then this is just a more extreme version of an old pattern. Decide if you want to change this pattern (if it exists). Or if this is totally new, is there something new going on in your relationship? Some new stressor?
And yes the consequence of your decisions may be the relationship ends. But it would be your decision, your thoughts and ideas about what you want.
Oh, as someone who had a difficult MIL, tell the mother you are no longer discussing your relationship with her at all, ever. Her opinion is not wanted or needed and you will not listen to it. If she tries, leave the room or hang up. You can't control your girlfriend's inappropriate inclusion of her mother in her business but you do not have to accept her in your business.
Hey, if the relationship ends, you get to have nothing to do with the mother!