Getting over ex

Long story short my girlfriend of 3 years broke up with me through text in October. She spent November and December begging for me back. I said no not because I don't love her but she broke up with me to find independence and to see what she wants out of life. I didn't think it was fair for either of us to hop into a relationship again like that. I certainly didn't think the door was closed. However I found out via facebook she has a new boyfriend. A guy that has been "friends" with her even while we were going out. I'm moved on and feel I can do better. I am still hurt by my main issue is I works with both of them at a seasonal job. They are both coming back. How do you move on when you have to see your ex with a guy I don't really enjoy everyday? Leaving the job is not an option for me as it is my career versus their jobs. I am worried about how I can stay happy at my job.