Replying to Damn girls.

so i developed a relationship with this girl and things were going pretty well, like we both really liked each other and talked about going out, and everything was going good. but then she tells me that she is going out with a different guy. so ive given her the cold shoulder and we havent talked for like 2 weeks and i see her and well im starting to feel like a jerk and she stills calls and seems like she wants to start up again but she is still going out with the guy... so should i lose her, or keep it going?