This doesn't sound like the story of a person who finds it easy to slip into practically any social group, lol.
I have to agree with WK. You have to approach people without preconceived notions of who they are and how they feel about you. Just be open and talk to people and if they don't start talking to you right away, then just sit, be present and listen. Eventually you will find a place to join in the conversation and often that's all it takes. If they aren't talking to you, it doesn't mean they don't 'like' you. It means they don't know you and are probably feeling just as hesitant as you are. I understand how it feels to be the only person in the room who is not a member of the clique, but you really need to draw on that self-confidence at those times and present yourself. And if they don't like you, well, at least you tried. Your girlfriend would appreciate that.
And I think leaving the party the way you did was pretty, I'm sorry, immature (even for a 19-year-old, yez I checked!) and, if I were her, it would be a huge turn-off for me. But granted, I am much older than you and probably have different standards in that dept. But you ditched her and you ditched her in front of her friends. Bad scene.
Your best bet at this point is to be completely honest. Like Skafe said, the ball is in her court. You could have handled this situation better. But, we live and learn.
