Im confused, im in a this yoyo relationship and i dont know what to to about it?

I luv this gal, when we go out together she always try to spend more time with me and refuse to go home early, everytime after our date when I want to send her home, she will ask me to go somewhere else or just drive around just to talk, holding hands and laugh, but I'm confused, few days ago I was on the phone with her and I told her I know you love me and without hesitate she say NO, I was suprised, I told her that was fast and she say it was her ego telling her to say no, because she knows I'm in a relationship with someone else, tell me how am I to decide whom to choose? I love her but her attitude make me think twice, she keep on telling me she don't want to be a bad person?