So I met a girl (lets call her K) about a month back, exchanged numbers. We talked, snapchated ect. for about a month. She wasn't to interested but i most certainly was. Halfway through I discovered RP and started implementing these ideas in my life. I imediately started getting with a girl from my school i thought was out of my league. But back to K, while using RP truths with her intrest started peaking. Constant texting, naked pictures. Long story short I don't whant her anymore and she wants me. Ive never blatantly put down a girl before, should I tell her upfront. If so how should I?

tl;dr I never told a girl no before, how should I