How should I win her back?

The most stupid thing I have ever done in my life is letting go of the person I love the most. I had been so preoccupied with my work that I forgot I had someone waiting for me to come home. My girlfriend was fed up and left me with no second thoughts. I let her because I always believe that if you love the person, her happiness becomes your happiness too. Then, I realized that the idea is absurd. It should have been, if you really love her, you won't let her go and you will fight for her. Those are the things I wish I did way back. When I met her yesterday at a foreign social, I realized I am still in love with her. I asked one of our common friends if she has been seeing someone lately. She said she did not notice anything, so I guess she's still available. I wish I'm not too late; but how should I win in her back? Advice, please!