Both me and my best friend had pretty serious girlfriends that started in high school and continued into our early twenties.
I definitely had thoughts along the line of "can I really live my whole life with this one girl.?" But me and my girlfriend never discussed it openly.
My buddy and his girlfriend did, though, and they decided to break up even though they loved each other very dearly. They both just didn't think they'd be able to ignore the nagging questions in the backs of their minds about what they might be missing out on, and how could they be sure either way with no perspective? They kept open the idea that maybe they'd get back together someday.
It was kind of a shame because the two pairs of us were really good matches. Each mate was very loyal and sweet and caring to their partner, much more than what you'd expect from teenage romance. But it is pretty much impossible for that first love to work out for a whole life, because even if you are that rare couple that is thoughtful enough to do things like treat each other right, then you're probably also thoughtful enough people to not be able to help but think the "Can this last forever?" Or "Would I / she be happier with someone else?" Or "What's it going to be like ten years from now never having boned anybody else?"
And those questions do just nag until something gives.
For a little more perspective, my buddy and I are still friends. We're 35 now. Neither of us ever did get back together with our old girlfriends, I image that at some point we all had seriously considered calling up our ex hoping to find them single and missing us, but I never did. I think I didn't out of fear that she wouldn't be on the same page.
I'm married now, to a girl that is amazing in general and terrific to/for me.
So if there is any moral to my dreadfully long story, it's that it will be painful all the time at first. Then painful in short bursts for a long time. Then it'll be okay. You'll date some turds, you'll date some crazies, you'll be the turd, you'll be the crazy. If you pay attention you'll gain perspective, you'll learn who you really are and what you really want.
Then in 15 years you'll be up with a stuffy nose and tell some kid your story.