Disclaimer: Caspian and Susan belong to C.S. Lewis. Their romance belongs to Disney.
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A part of her died when she turned to join her siblings to step into the portal leading them back home. England never felt like home to her. There wasn't a single day that went by over the past year that she didn't long to stay in Narnia. Life in England held no place for anyone who believed in magic or fairytales. Peter had told her that he believed that magic and the talking beasts had existed once in their own world, but magic had vanished over time, revealing itself only to those pure and innocent enough to believe. All legends and myths have basis in reality afterall. Susan knew that it was real, not her imagination.
The only life she anticipated in England would most certainly be dull in comparison to what she could have with Caspian. Why couldn't Aslan see that? Why did everything always have to be his way, when everyone else was hurt in the process? She and Peter learned all they could here? How could that be when they were allowed to stay much longer the last time? Also their mother and other adults had told them that learning was something that everyone did their entire lives. She hated the choice he made for them, but it was also dishonorable to defy him. Still, she silently pleaded that he would allow her...them...to stay. No. There was nothing in England for her anymore. Ever. Of course Aslan wouldn't know that since he had never been in their world. She wasn't about to settle for the dreary buck-toothed boy in glasses from the newspaper stand. Of course, with her luck, he'd end up as her husband. Getting married and raising a family would be expected of her. But she couldn't possibly marry anyone she didn't love.
She had found the other half of her soul in a place that didn't exist even in stories in her world, and here they were being torn apart for eternity. Susan knew deep down this would happen, which is why she was scared to get close to him and did her best to keep her distance. But fate had other plans. The taste of his kiss burned her lips. She could never kiss another with the same passion and desire.
Susan could feel Caspian's mournful gaze searing into her back. She knew he had died as well. She held Peter's hand and squeezed it in search of strength, hoping he had comfort to offer, but knowing he had none. She sighed and blinked back tears as they stepped into the portal.
Now all four Pevensies stood in the train station, as though Narnia was but a daydream and no time had passed at all. Soon it would be nothing but a dream and a far away memory. Perhaps it was better that way.