My mom and dad met when they were about 13 and went back and forth for the next decade of their lives. They were young and afraid of the future. Now, they're married, have 4 children, and just celebrated their 22nd anniversary.
I think all relationships have to end in one way or another. Maybe some space, distance, or both. Nothing can stay 100% great all the time. What really matters is that two people are willing to put in the same amount of effort to meet a common goal.