Please help... Urgent?

ok so basicly i go out with this girl, but i dont want too, the reason why i said yes in the first place is because when i said no, she cried her eyes out and i felt really bad. So i was trying to talk to her about it but she was so happy i dident want to upset her again. I do fancy another girl, but no cos i go out with this girl, she now flirts with other guys, its really painfull inside :\ people say get in there, but i dont feel anythng for her. Im talking to her on facebk right now and i dont no how to say that i like someone else?thanks for your help.ebk right now and i dont no how to say that i like someone else?thanks for your help.