Whatever you two did that night, was really that bad that it has allowed this to go on for so long?
From what I have got from this, you and your girlfriend both really really love each other and have stuck with each other throughout. That is what really matters!!
Am I right in thinking you and your girlfriend are always there for each other and great support for each other?
I hope you dont mind me saying this, but it seems all is ok til your mother talks to someone, then that someone is against your relationship?
Maybe you should sit with your mother and find out exactly what the problem is and how you can work on it, with your mother, to fix it (other than finishing with your girlfriend!) Make her see how much you two love each other, and make sure your mother realises SHE forced to move out with her actions.....not your girlfriend!