This story is based on chapter 69 of Wish-Chan's story The New Trials of Card Captor Sakura and Friends. This story has what I believe Sakura to have thought in the time up to the fight between Sakura and Syaoran.
Flash back
Sakura looked at the tall boy as he practiced his sword drills. Syaoran was so intent on his target, which was a master swordsman created by Sakura using the create card. Syaoran matched his foe at every clash and effortlessly dodged each blow. A person with any less skill would have been seriously injured. The battle was so intense that he never noticed that three girls were discussing him. "Syaoran kun has certainly developed in the last few years. He was definitely skilled when he first came at age ten, but it still is a night and day difference from then," Sakura commented with a smile. Even with her earlier words, she winced as Syaoran's foe slashed at him, and then sighed in relief seeing that Syaoran had easily dodged the attack.
"When Syaoran was home in Hong Kong, he spent every spare moment in training, and the Great Elder is a severe trainer, even in his low health, Syaoran was dead on his feet during supper every night," Meilin informed Sakura and Tomoyo. She continued, "Even so, he managed to earn high marks in nearly all of his classes. Plus, he was a soccer star. All of the girls swooned over him."
"Were you usually included in that group, Meilin?" Tomoyo kindly teased. Meilin blushed and said, "Oh hush, Tomoyo. I have Kai kun, now. Even if Syaoran would be a better boyfriend, I really do like Kai."
Tomoyo smiled, "The heart is not always rational. Sometimes it chooses one who has already chosen another." The smile waned as though she was considering one of her own experiences. When Sakura heard this, she thought of the original Dark Ones. Chang Risa had given all she had to Amimaya Hayashi a baby, but it still was not enough. When Risa found out that she was pregnant, she also discovered that Hayashi love Li Shulin. The tragic story ended with Risa's suicide, but the effects still stung the descendants to this day.
Sakura's thoughts were interrupted by a thud as the created swordsman collapsed to the ground and Syaoran put his sword to the throat of his foe. Sakura stood and made the defeated man disappear. She handed Syaoran a bottle of water and a towel to wipe his sweat. "Good job, Syaoran-kun. Are you hungry?" Syaoran blushed and nodded. He then sat and took a Bento from the large picnic basket. Sakura followed and sat close to Syaoran.
Current day
Sakura sat in Clow's old office and considered Syaoran's challenge. She had started to realize her feelings toward him. Although she did her best to forget Syaoran, it only resulted in more pain. Then Syaoran stole the Sakura cards. The rest of the Alliance still believed that Syaoran was on the dark side, but Sakura knew for certain that he was the same Syaoran that he always was, and she knew that Syaoran would always be there for her. He was such good support when the two of them had visited Subaru's grave. Tears came to her eyes when she thought of the foundation he had set up. So much had happened between them, but Sakura knew, at least for the moment, that Syaoran was special to her, even necessary for her to live. Even knowing this fact, Sakura had her doubts.
Sakura remembered the movie quote, "A fish may love a bird, but where would they live?" Syaoran's world was much different from her world. Syaoran could rise to infinitely high places. With the Great Elder's death, Syaoran could even rise to that position. Sakura, however, was content where she was. Yes, she was the mistress of the Sakura cards, but the cards were out of her hands, now. Anyway, both the proverbial fish and the bird could rise to great power, but only in opposite directions. The fish and the bird could never dwell together for long. Only one can survive. With this challenge came in the clash of the worlds. She remembered that day in the park once more. Her heart was indeed illogical in choosing Syaoran.
Just like Risa was unable to make her relationship with Hayashi work, would Sakura be unable to make her relationship work? After all, as much as they tried, their parents were unable to make their relationship work. No, Sakura again determined to not allow fate to have a hand in unraveling Sakura and Syaoran. If Sakura and Syaoran found others, it must be based on their decisions.
Although it was not stated, Sakura could guess that it would be a duel to the death. Either Sakura or Syaoran, or both would be dead tomorrow. Sakura did not want to die. Yet, could Sakura bear to kill Syaoran? No, how could Sakura, after spending so many hours and magic healing Syaoran's back after he sheltered her from the Whip, put a sword through his back? Or, how could Sakura pierce Syaoran's heart when her own heart was so closely intertwined with his heart? If she killed Syaoran, she killed herself. She tried once to live without Syaoran, but she knew that she could never succeed. She didn't want to try. No, it would be far better for Sakura to perish. Yes, it would be painful for her friends and family to accept, but it was the only way. As for the Sakura cards, Sakura trusted Syaoran to take care of them. As hard as was to accept, if it came down to it, Sakura would have to breathe her last. Sakura and the alliance approach the park. As she suspected, it would be a duel to the death. Syaoran, of course, used his sword; even if he was able to, the Sakura cards would never fight their mistress. Sakura created her own sword with the more recent cards. The two fought for a while. Sakura was a good match for Syaoran, but Syaoran was more skilled. Syaoran now had Sakura in a similar position to the created foe from the park so long ago. Although it was good enough to assume victor back then by merely getting a foe into that position, for this duel, as long as Sakura could still draw breath, the fight went on. Sakura saw the hesitation in Syaoran's eyes as he held his sword close to Sakura's heart. She could see that could and perhaps would still back out. Knowing that it was she who must lose, she rushed forward, allowing the sword to plunge deeply into the heart which had so much love to give. The body fell into Syaoran's arms, allowing him to hold her once more before the beautiful corpse vanished in a gust of Sakura petals. With this act, the Sakura flowers bloomed on Sakura's birthday, but as the body disappeared, the trees were left bare, as if in mourning.