In her dreams, Serenity returns to her earliest days with Endymion. Every time she had gone to Earth, her stomach would be filled with butterflies, the result of her fear that one day Endymion would betray her. But he never did. Nothing ever happened, nothing that would even remotely indicate that he was less than loyal to her, and slowly the butterflies transitioned from those born of fear to those born of love.
And apparently, he had felt the same way because one day he had kissed her, slow and sweet. And that, more than anything else had doomed her. Before, she could have left Earth behind, the memories just those of a daring adventure with a slightly unconventional friend. But after the kiss, it became love, and love is far harder to leave behind.
That kiss is a nightmare now; it repeats over and over in her dreams, so sweet and pure, and then she wakes up and remembers that Endymion is gone. He is engaged to an Earth woman, and he can't throw that away for his fling from the Moon.
Serenity wonders if perhaps she was just a distraction, a fun time for him until he had to get married, and now that that is happening, he can throw her away. She would like to think not. She'd like to think that Endymion's emotions had been real, that he had really wanted to be with her. But maybe, she was just a bit fun in a bored prince's life, a particularly exotic mistress who in the end meant nothing.
It's easier almost, to think of it like that though. That she is nothing to him. Because then he never loved her at all, and she can hate him for deceiving her instead of pining after him. But in her heart of hearts, she can't believe that and knows it isn't true. Endymion looked as devastated as she felt that day, but despite their feelings, their affair can't go on. Guilt would destroy her if it did. Even if they are in love, it would still be wrong for them to destroy his marriage.
So instead of running back to Endymion and begging him to come back to her, she lies in her bed, tears leaking out of her eyes, dreaming of kisses and regretting them upon waking.