Fucking hell man, if she's being that persistent, she's cheating. No way in hell someone your age is continuously gonna say no to sex, unless she's getting it somewhere else. This is the time to tell her it's over and that if she doesn't wanna give it up, then you'll find it elsewhere. Remember, pussy is pussy. You can have a friend with benefits if you want, with no emotional connection. Either way, good luck and make the right choice.