Just a response to the last point, if you don't date now you never will again: I'm not sure that's true. Women are well aware that people want things from them, and a little speech to the effect "if we date now, you will be choosing me because it is familiar, and though I don't want to say I wouldn't like it, it would very clearly mean that you would miss out on a lot of the stuff that happens in the first year of college -- meeting totally new people, learning to be on your own in an alien environment, making your own decisions from scratch, etc. If you let me be that crutch for you I know in my gut you will never forgive me, and I don't want to do it. I'm torn because I think you're amazing, but if we're made for each other we can figure that out next summer" could go a long way. And then just quietly stick to your guns. You know her well enough by now, I assume, to say this in a way that she will be impressed and flattered (after all, it *is* impressive and flattering) rather than just bursting into tears and claiming you never loved her. The reaction depends on the sincerity of the delivery. And to be honest a few more clandestine dates never hurt anyone provided that the ultimatum is still in place. If anything it will allow you both to enjoy the various thrills of transgression and decadence.
When in a difficult situation involving another person, a reasonable general strategy is often to articulate your worries in a convincing enough way that the other person has fair warning to act on them. Let her bear some of the weight of the decision -- you don't need to keep your angst entirely to yourself -- after all, she probably senses some of your ambiguity.
