I feel for you, and have been in a similar situation myself. I messed up even further by trying to keep everyone happy.
Now that I'm older, I would do what was best for me, and my partner, (and quite frankly, F*** the rest). People who really care about you have your best interests at heart. If they are willing to jeopardize your happiness, it means that they don't. Plain and simple.
You need to be as independent as possible so that you can call the shots in your own life. Stand up for yourself and your girlfriend, even if the going gets tough. Frankly, the way some mothers hang on to their sons is just kind of creepy. Are you the only son?
The other thing I would say to you is try to avoid emotional discussions with your mom if you can - they lead nowhere. Just state your case and don't get dragged in to the drama if possible.
I wish you all the best. Hang on to every bit of happiness that you can
