The fact that you don't stutter with your GF is a good sign, it means that a lot of it is psychological/mental, most likely the anxiety and lack of confidence that comes with the stutter. There are no easy way out of this but the good news is that with hard work, it will get better. Find a way to increase self-confidence, step by step, then the anxiety will progressively decrease. Start with small social interactions and increase over time. There will be difficult moments, but with perseverance, you will improve little by little. When I was a child, I couldn't say my name. Now I give presentations to the Executive Committee at work. I am the proof that its doable. We will never be 100% fluid in our speech, but we can be much better. It does get better, I promise. I hope this helps