Hi,
I'm sad to hear that she just left and didn't tell you why? Maybe she didn't have the courage to own up to her feelings; possibly she wasn't getting from the relationship what she was seeking. Or she found someone else and again didn't have the courage to tell you.
We cannot control how other people choose to act or deal with things, what we can control is our own actions and how we choose to deal with things. I think the only way for the pain to stop is for you to get closure. However you choose to create closure for yourself is up to you.
Sonia