Help I'm in love with him!! What can I do?

My name is roxy, I'm 20. I met a guy not too long ago in a chat room and we hit it off. I got his number and we started talking over the phone and texting. About a month ago I got a call from this girl and she told me that she was his girlfriend of 3 years and they just got an apartment together. She told me she was in love with this guy and wanted to marry him. She asked me if I'd stop contacting him and I said I would. I haven't stopped yet. I fell in love with this guy. I want him to leave her and come be with me. He doesn't really talk to me anymore. I try to call and text him but I hardly get a response back and its usually short and brief. He says him and his girlfriend are celebrating their anniversary on Oct. 19th and he is thinking of asking her to marry him. I can't let that happen. I want him to be with me not her. How can I get him to leave her for me? How can I stop him from asking her to marry him?