I don't understand planning to get engaged. Either you're planning to get married (in which case you are engaged, whether there has been a ring and a bended knee moment or not) or you are still talking about it in the abstract (in which case you are not engaged)
This, exactly. And yes, too soon, and by the sound of you, too young also -- and I say this as someone who got together with the man who is now my husband when we were both 19. But we met at university, didn't tie one another down with discussions of wedding ceremonies and engagements, spent time living in different countries, pursued our studies, and didn't get in one another's way. We realised the strength of what we had late.
What else do you want to do with your life? I think you'd be crazy to tie yourself into a very new, exclusive 'engagement on the cards very soon' relationship at your age, and when you are about to head to university. If you're doing it right, university will, as well as your studies, give you the opportunity to explore entirely different ways of being and bring you into contact with very different kinds of people. My now-DH and I were able to explore them together, but too often I see women students in particular with LD boyfriends cutting themselves off from opportunities, living on the phone, or running off to spend all the weekends with them.