I do not think breaking her heart is the right way to deal with this...just sounds like it would cause her more problems. You mention you already somewhat broke it off, why not talk to her about how you feel instead of just trying to break her heart so she leaves you...If you don't want to be in a relationship with her then the best thing to do is clearly say that...flirting with another girl or going as far as dating them if you're technically dating this other women is not something that would end well for anyone involved I don't think.