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I think you are overthinking this, simply put she is lying and Sai knows this by her false smile.
Not exactly, I think Saku-chan sums it up best:
I think Sai's interpretation of Sakura's response was a little odd. I don't know though. I did not look at the raws, so it could be a translation thing. I've reread several times and I think that he meant that her words were what she sincerely WANTS to feel, but she doesn't feel this way and that's what's making her sad. To extrapolate a little, I think that she feels that she cannot forgive Sasuke like Naruto apparently can. She may also feel as if she has put a burden on Naruto with the promise of a lifetime for no reason. Anyway, I love that Kishi is no longer sidelining her feelings.
Sakura truly wants to believe that everything will be OK, and she wants to believe her own words, but she can't shake that uneasy feeling that is growing within herself. Things are not adding up, and she knows it.
As much as Sakura would like to go back to the old days when Team 7 was whole, deep down she knows that everyone has changed - herself most of all. Orochimaru said it out loud; people change.
We may well see Sakura and Naruto fighting against Sasuke in the near future. Both Naruto and Sasuke want to save the villages, but Naruto wants to heal wounds, while Sasuke wants to destroy everything and rebuild it all.
Sakura's smile was the peak for me.
And it seems like Tsunade didn't expected Sakura to be able to summon Katsuyu...
Personally, I can't wait for Tsunade to witness Sakura's growth on the battlefield. 
While it was unsettling for Oro to be comedy material, we do at least get to see that he hasn't completely changed. Before, he always wanted to be the wind that would drive conflict and change the world. Now, he wants to sit back and let Sasuke be that wind. He believes that Sasuke will change the world, and he wants to watch what happens.
Of course, we know that Naruto Uzumaki will be the one who really changes the world for the better. 