Sakura could be a bit angry at first. She has grown and become more independent during his absence. When Sasuke shows regret, she may lash out, expressing all the pain and frustration she felt during his long absence, but deep down she still has feelings for him.
One possible reason could be that he finally realizes Sakura's unwavering love for him. Throughout their time apart, he might have come to understand how much she truly cared, and now he regrets not being there for her. Also, he could have seen how she grew stronger on her own and feels he missed out on being part of that growth.
She might be initially heartbroken. Sakura, being in love with Sasuke, would surely be deeply affected when he neglects her. She could become withdrawn herself, losing some of her usual cheerfulness.
Self - discovery is also a theme. They come to understand themselves better through the regret. Sasuke might realize that his pursuit of power wasn't as fulfilling as having a relationship with Sakura, and Sakura might learn to value herself more after years of pining for Sasuke.
Well, Sasuke may regret cheating on Sakura in fanfiction due to various reasons. Maybe he sees how much pain he has caused her. Sakura is a strong and loving character. She dedicated a lot to their relationship. When Sasuke cheats, he not only breaks her heart but also the bond they had. He might then look back and think about all the good times they had and how his actions have destroyed it all.
Sakura would probably be initially very angry. She has strong feelings for Sasuke, but kidnapping is a very extreme act. So she would likely fight back, using her medical ninjutsu skills and combat abilities to try and escape.
Another reaction could be that Sakura tries to follow Sasuke. She believes that she can change his mind or that she can be of help to him. She might leave her own responsibilities behind for a while to chase after him, thinking that their bond is strong enough to overcome whatever is making Sasuke leave.
On the other hand, Sasuke could be very stoic about it on the surface but internally be in turmoil. He might start to analyze his own actions and whether he had missed any signs from Sakura. He could also be the type to immediately start thinking of ways to win Sakura back if he realizes he can't stand the thought of her with someone else.
One reason Sasuke might regret cheating on Sakura in fanfiction is that he discovers the depth of Sakura's love for him was far greater than he thought. He took her for granted and when he cheats, he realizes how wrong he was. Sakura was always by his side, supporting him, and he just threw it all away. So, the guilt starts to eat at him.
He might be initially confused. Sasuke is not used to Sakura ignoring him as she has always been somewhat persistent in her attention towards him. So, he may be at a loss as to why she is suddenly acting this way.
Sasuke might initially be shocked. He has always seen Sakura as someone connected to Konoha, so her leaving would be unexpected. He could be concerned about her safety, especially if she leaves without proper preparation or protection.