Break up with the girl first, because that's cheating.
And it'll make her feel really shitty after.
Be all honest with her. Say you like her, but need more time with her and don't want to pull her away from her for filling her full potential, so you think it'd be best just to end it, so you can both see other people.

Say you wanted to call it off before it got too serious, but you really do like her in that way (make that clear so she's not all "OMG NOO." teenage girl impression) and yeah, I think just carry on flirting gently, don't be all "You see the fire in my eyes? The fire of a thousand burning suns, My passion alight for your love!" or anything, it'll get weird.

Play it cool (;