K, a girl I met at a party, and I have been talking. When we met, K was giving obvious signs that she was into me, and the next day we began snapchatting nonstop. Shortly thereafter, we started texting. Typical "omg we have so much in common" honey moon phase flirting. Convinced her to break up with her bf, and date me. Her bf is the cousin of my best friends gf, so we wanted to keep things fairly low key. Word got out, K told him everything, and then after a week or two of disinterested texting, K gets back together with her ex. Whatever, move on, didn't text her since then. This morning she texts me and asks how things are going, we exchange a few texts, and I stop responding.

I realize this is probably the time to just next her, but I would like to fuck this girl, what is my next step?

Let me know if more info is needed.

tl;dr I meet taken girl, woo her with cockiness, she breaks it off with her bf, and then after a short interval gets back together with him, but I want to bang her.

Thanks for any advice!