Disclaimer: Tamora Pierce owns these characters; I do not.
They had something beautiful once, the sort of romance that bards would sing about with wistful tones as even they had never experienced anything so powerful. The two former lovers still can picture and remember it, as they haven't changed that much from the young people that they once were, when they were so unsure of their place in the changing world. They can see it in the way their eyes meet, sometimes, and every so often they'll dream of the times that they spent together, the nights that they spent so deeply in love.
But the magic is gone, just as surely as it arrived, taking them both by surprise in the unlikely turn of events that was their love. After all, it was ill-fated – a prince of the realm and his squire, a girl that hid her true gender to become a knight? The people would never have accepted her, especially so soon after women could become knights, so perhaps it was for the best that they stopped seeing each other.
They have found other partners, now, ones that make them happy in the way that their first love never could. He has found a beautiful woman and a wonderful queen, one that he loves with almost all of his heart. She's wise, and she can detect a glimmer of affection that goes beyond the lines of platonic friendship when he smiles at her, but she doesn't comment, for she knows that he's devoted to her.
She found love with the rogue that always cared for her, that was always utterly dedicated to her, even before the prince was. He was the one that consoled her about the prince, the one that the prince had thought was only a replacement for himself in the old days before he saw them together, before he saw the smiles they gave each other, radiating love without a word.
They have both moved on. They lead such different lives now, it's hard for those that knew them when they were lovers to picture them together now; he's the king now, and she's a knight. They're both legends, but it's always better if legends don't love other legends. It's a matter of glory and competition, which would have inevitably ripped them apart. They are made for fighting. They aren't made for romance, not with each other.
It took them a long time to realize that sometimes they have to yield to convention when the fate of the realm depends on it; there are some things more important than their own happiness. At first they thought that the only thing that kept them apart was the fight in the desert, but it was so much more: the tension, the knowledge she had that she could never be what he needed in a queen, his knowledge of that fact that he tried to hide, that he tried never to think about. It was the fight that brought these tensions and problems to the surface, showing them before their own eyes that something wouldn't work, unless something changed. And it didn't.
They're content now to just be friends; it's better for everyone that way. They don't speak of their romance much with one another, it's the one subject that they actively avoid, the thorn in the leg that they don't want to pull out for fear of causing more bleeding. It'll only make things awkward between the king and his champion, the two best friends, and neither of them want that.
Sometimes regret will cloud over them, will make them think about what might have been, what their lives could have been if they had worked through their problems rather than push them aside. But they know, in their hearts, that their relationship was doomed from the beginning; it was why they fit in all the hurried words of love and promises they couldn't keep, so they could make themselves think for one more night that they might have a chance. And they knew that they could never spend the rest of their lives together in marriage, however much they wanted to.
Sometimes lovers don't have happy endings. And sometimes it's for the best.