Regret is the most useless of human emotions. Sakura feels useless, and stupid, too.
It could have gone differently, if Ino hadn't felt the same. They would have each had their own guy and been happy for each other. But now, it's all behind them, even if they're both still living with the consequences. Now Sakura can look at it calmly, rationally, and she knows Ino can't be blamed any more than she herself.
But she can't believe it could have been some other way, because that's too painful. She can't ever actually admit that it was foolish of her, because that would mean she doesn't love Sasuke. That would mean she gave up Ino for nothing but a daydream, woken from abruptly, to find the real world in chaos around her fleeting illusion. She can't believe how stupid they both were.
Can't believe boys and petty things like that drove them apart.
Can't believe her own nearsightedness.
Can't believe how horrible she's been - to /Ino/, Ino who believed in her, Ino who helped her up and gave her a world she could live with, Ino who was going to be her friend forever. Forever, until there was a boy.
She sees now, boys really aren't all that important. Compared to Ino, now she sees that Sasuke isn't worth any of it.
Now, when it's too late...now, she sees.
Sakura can't believe she thought she loved Sasuke more.