String her along, and then tell her you are seeing someone, slip it in that, you are seeing a hot brazilian model who studying law at university.
Seriously, the only reason why girls these days can get away with shit behaviour, is because guys are too desperate to have sex with them. You seem like a good guy, I'm not saying "Niceguy" i mean genuinely a good guy, realise your worth, and don't resort to the easy option, as i can tell you are contemplating it, being on this forum.
She will do two things to you, if you hook up or text her back showing too much interest.
1) She will flake on you, reject you somehow, stop responding, so it gives her the satisfaction of being wanted by you, and that she can have you back whenever you want.
2) She will have sex with you but after she will reject you, maybe not first of all, maybe it will be a few month relationship, but that will be it, probably cheat on your ass.
This has nothing to do with your character, it's simply the brutal dynamics of mate selection, especially in today's pointless consumerist culture. My best bet, is too tell her that you have an amazing girlfriend that you are happy with, and tell her to go and do one.
