Matthew, I have grandchildren the same age as you and your girlfriend and I will say to you what I would say to them. You appear to have a strong sense of entitlement and along with your unwillingness to accept advice or criticism, that is a very unhealthy combination. There are many young people your age and your girlfriend's age who do not have ideal home lives and just get to it and have to find a way of reaching their goals if that is what they really want.
Grow up. Get a job. Get a life. And stop expecting everyone to dance to your tune. You are in for a rough ride in life if you continue with your current attitude. As for your girlfriend, it seems to me that you can't fend for yourself never mind another vulnerable human being.
And don't bother with the throwing the toys out of the pram. With 16 children, I've heard it all before. Water off a duck's back.