If this were real life and the story continued somehow past the final moments in the book, college would eventually start for the two of them and Lara Jean and Peter would either try to make the same college work, which would mean one person could regret the sacrifice, or they would go to separate schools and try to make distance work.
Either way, most relationships (not all but most) that try to keep the spark between high school and college fail.
It doesn’t really matter how To All The Boys 3 ends and whether or not it follows the same trajectory as Jenny Han’s third book in the series.