Man up, have one last go. If it's a no, drop the issue and go get drunk. You've another year to two years at university left for heaven's sake, that's plenty of time to find someone that isn't a vapid teenager... may have more brain, shared aspirations, mutual interest (both in each other and other things) etc.
Plus, although 18 is just about within your non-creepy-age-gap zone, you'll still have been postgrad for 2-3 years by the time she's doing finals, should she also go to college, and there's a very low chance she'll end up at the same one as you. 5+ years of a very fragmentary relationship... if you think the fire burns strong enough on your side, and might on hers, it could work. Otherwise - forget it.