From what you said it sounds like its just your pride hurting dude.

It sucks and it hurts, but it won't last forever.

Unfortunately there is probably little you can do to get her back now. And as tony said - you don't want to, if she is playing games etc.

You have two options.

1) Never see her again - fuck her - you are the most important person right now. If it too painful to see her then don't. If she asks about it, you don't have to be all wussy and needy. Just tell her how it is and don't appologise for it. Then move on.

2) Remove yourself from the situation just long enough to take an objective look at it. What are the facts, and what is just in your head? I've been in very similar situations. A girl I was with dumped me and started going out with another guy very soon after. And it fucking hurt. I wanted her back. And actually if I'm honest with myself I wanted to make her hurt like she hurt me. But if I could go back and talk to myself I would say 'Ben. She dumped you because you did something wrong. You wern't attractive enough. You wern't relaxed enough around her. She is young, and at this moment in time she finds this other guy more attractive. So it's YOUR FAULT. Not in a negative way, but in the best positive way there is, because once you realise that it is your fault, you have control. YOU CAN TAKE CONTROL OF YOUR REALITY. So stop feeling shit and feeling anger towards her and some chump. And think 1) What happened? 2) What will I do differently next time so that it will never happen again?'

I don't know it this applies to or helps with your situation, but the important thing is that you learn from this moment so you can make positive change.