I want the girl back i finsihed with but shes hooked up with someone else, please advise?

I ended my relationship with my girlfriend over xmas coz i was confused.(I had only just come outta a long relationship previous 2 her and jump straight with her , shes the reason i left ) anyhow i then realised i had made a hugh mistake but she just ignores me, but i keep at every chance just trying to get a glimpse of her ( we work 2gether ) but i found out over the weekend she hooked up with someone else on Friday. Im just ignoring her now.. what should i do.