How to get my pissed-off girlfriend back?

She and me were on extremely happy, healthy relationship for 2 years in our college times. Two months ago, we got job on two different IT companies (in same city, our offices are just 5 km away). During last months, she lost interest on me and one week ago, she just told me that she didn't feel for me anymore. Though she is enjoying her life with her new office people, I know from sources that she is not dating anyone else. However, a man in his office is trying to impress her through showing extreme care. She is saying that she is not interested in being in relationship with him as he is too much old for him but she is hanging out with him more than me - that made me insecure. Being madly in love with her, I got desperate for getting her back and called/texted her frequently, told not to hang out with that man and come back to me, which eventually pissed her off. Yesterday, she told me not to call anymore. What should I do to get her back as soon as possible. She is amazing and life is too much hard without her. :(