To be fair, Sasuke was always this way, as he said himself. The thing is that instead of accepting it, he denied himself any emotional attachments so that he could grow stronger. Throughout the entire series you hear him talking about cutting bonds and everything because he still believed that being alone was what made him stronger.
So Sasuke didn't change, he just accepted that what he was doing wasthe wrong way to go about things.
Sakura's forgiveness is understandable because she always wanted to save him, just like Naruto. I've said this before and i'll say it again, If Naruto wasn't powerful, he would be in the same exact position as Sakura. Though, what really makes it seem like it sucks is the lack of development for the relationship between her and Sasuke, or her character in general. I'm sure Kishi wanted to portray her having a real emotional attachment to Sasuke like Naruto, but he failed...horribly.