Today's Trope: Teenage Romance

I've written a few YA stories and a few romance stories, and it's got me wondering something. Since most people don't end up staying with the person they dated in high school, how "forever" do you have to make the relationship seem when writing fiction?

For example: in the YA series I'm currently querying, my main character starts dating during his junior year of high school (book 2). That relationship goes poorly and he ends up dating someone else his senior year (book 3). That relationship goes very well and they're still together when they graduate. But this isn't the person my main character is going to spend the rest of his life with. He meets that person after college (not sure what book number that will be). But my MC loves his book 3 LI. He's devastated when they break up in book 4. It's just a fact of the human condition, most high school sweethearts break up.

I recently started a YA romance one-shot, and I'm having more trouble with this one. The problem I'm coming up against is that I don't want to think about my MC and his LI breaking up, EVER. They're great together and they really love each other. But they meet when they're 15. If this were the real world, what would be the chances that they stay together for the rest of their lives? Pretty much none. It's fiction, so I can pretend they stay together forever. 

But I'm also a bit of a realist. (Not a helpful personality trait for a writer.) I want to write my romance books with a hint as to what comes after "the end". For an adult romance one-off I recently wrote, I ended it with a proposal. That's about as close as we can get to "and they lived happily ever after" in modern fiction. I can't exactly do that with a couple of teenagers. Maybe showing them going off to the same university? Do an epilogue of them 10 years in the future when they're having their first kid? Ugh. Then there's my brain telling me that there's no way these two would stay together all through high school and college without at least temporarily breaking up and trying their own things; dating other people or moving away for a few years. That can be part of healthy development.

What to do, what to do, oh, what to do? I guess I just write the story that's living in my head and hope the readers can extrapolate the kind of future I envision for the characters.