You know, I totally thought I had everything down about relationships when I was with my first serious boyfriend, and that we were meant for each other and would be together forever.
But being through a few relationships, good and bad, and breakups, I know there's a lot out there I DON'T know. So I'm happy just seeing where the road takes me, no expectations or goals or planning. If you're meant to be with someone, it just happens, without fretting or thinking ahead to any major extent... at this age.
We're all so young, I think it would be better to approach a relationship with the mindset of "what can I learn from this person?" and "how will this relationship change me for the better?" rather than "when will we marry?" or "what will happen to us in three years/two years/a year?"
Even if you don't get married, every relationship is a stepping stone to the right one, and every single person should be more concerned with learning, knowing themselves and being happy rather than forcing anything to work.
The best relationships come about that way.