She waits, and wonders why he left...
She sits here on that bench, in the shade of a tree, beneath the setting sun where he left her and she watches, waits and thinks her gaze drifting to the spot where he vanished.
At first, she did not comprehend why, but now she thinks that conceivably he left not just for revenge but also for peace. Peace of mind, his remaining sanity, because if that man still breathed, the possibility of it happening again existed.
Even if she has never lost as much as he has, loss is something she does understand for in leaving she lost him and that makes her feel so empty inside. She knows now the difference between needing and wanting, he taught her that and she's thankful.
Now she realizes that it was never a case of wanting Itachi dead so much as needing him dead, because a man drowning in the past has no future. With understanding comes acceptance, and while she disagrees with his choices, his methods, she accepts that for Sasuke to be able to truly live Itachi must die.
However, as she waits she trains, she needs to get stronger so that when it counts they can all lean on her. She trains because she gets tired of waiting and she will become another Gai clone before she gives up on Sasuke.
She trains because Naruto has a promise to keep, and she will be damned if he will keep it alone. Now that she is older she knows better, but that will not stop her from trying, from pressing the thorn into her chest. Sure, it hurts beyond all reason, but she does it anyway because she is Sakura and she loves him, Shannaro!
Kudos if you caught the Thornbirds reference.