Tell yourself a new story and stick to it. Here's a quote from a book called "The Vortex" that I recommend for any situation in life. "A belief is only a thought that you keep thinking."

Try this for the next three days:

1. Forget about her. No contact. Don't think of her at all unless it feels good.

2. Do what you love. Do what makes you feel fun, relaxed, alive, optimistic, passionate, and filled with joy. It could be anything as simple as playing a video game, working on your music, or even going camping with friends. Do anything that will make you think at the end of the day "Wow, today was a great day!"

3. When you're in those feel-good moments, think on what you appreciate about yourself. Think on anything that you love and appreciate, but also especially think on what you love and appreciate about yourself.

Your belief that she's seeing someone else is actually the belief that she's into someone else and not you, which is actually the belief that you are not good enough and not worthy of the love you truly seek. Energy flows where your attentions goes. By taking time to enjoy your own life and appreciate who you are unconditionally, not only are you expanding your own pure positive energy flow (to borrow from Abe-Hicks), but it's so much easier to believe in yourself and to believe that you do attract love effortlessly (my own experience).