"The trouble with changing the world is ... you don't. Not all at once.
You just inch it forward, a bit at a time, and watch it slip back, like the Greek guy with the rock.
And you hope that when you're done, you've moved it up a little, changed it just enough. "
L A S T G L E A M I N G P A R T V, 8x40
Or, Buffy's Relationship With Redemption
There are really three main areas in which Buffy refuses to forgive herself. The instance of making Angel lose his soul and I Only Have Eyes For You, her arc in season 6 and her inability to forgive herself for her own depression, and her current arc in season 9. Since I Only Have Eyes For You is the subject of this essay and already discussed, we'll move straight into season 6.
Buffy's issues in season 6 are multi-faceted, but they all boil down to one thing: she doesn't want to be here. She doesn't want to live in the world because it's hard and it's scary and where she was? It was better there. So, it takes a long time for her to anchor to the fact that she's really here. And, by the time she does, she's done things she isn't proud of.
She's hated her friends for bringing her back. She didn't admit to it, she didn't vocalize it, but it's there. A part of her, a part that she hated for being there, hated them for not being able to take care of themselves. Hated them for needing her back so much that they would raise the dead. Hated them for pulling her out of a place where she was safe and happy and warm and loved into a place where she had to struggle just to keep living, when she didn't even want to.
She's been negligent of them to the point where Willow and Dawn self-destructed without her even noticing. She neglected Dawn in favor of her own depressive, self-destructive behavior, to the point where social services had to come. She neglected Willow to the point where she was so wrapped up in her own life that even when people tried to point out Willow's problem to her, she simply over-identified, took it as a personal attack, and deflected the problem until it had exploded and destroyed Willow completely.
She's treated Spike like absolute shit when he was the one person who really truly was there for her, when she had been the only person who treated him like a man instead of a monster before, when that was the whole reason why he loved her so much, and seeing herself now she saw someone who wasn't Buffy at all, someone who behaved like an animal. She used him, disregarded his feelings entirely and excused it because he wasn't a person. She used him to feel dead, to feel alive, to feel anything and pretended like he felt nothing the entire time she was doing it.
And it all boils down to hating herself and trying to punish herself for not being able to snap out of her own depression. She doesn't make allowances for herself because she just came back to life. She doesn't make allowances for herself because of what she's been through. She is ashamed of and loathes herself for the way she behaves, because she spends so much time convinced that she's not Buffy, that she came back all wrong and couldn't possibly be her, that she comes back to herself and realizes oh god, what have I done?
She tells Holden Webster in Conversations With Dead People that because of this, she is worse than any of them. Beneath them. Her friends, her family. Even Spike. She refuses to forgive herself for her actions, and more than that, for her superiority despite them.
And then in season 8, it gets worse.
In a way, the means through which Buffy realizes not to blame Angel for what he did as Twilight and as such forgives him for it is similar to their arc in I Only Have Eyes For You. She has to go through and experience it. Angel was under the influence of Twilight, and even though it was his actions, his decisions, Buffy realizes when Twilight influences her that it's not really entirely that simple.
She has sex with Angel when the moment before she'd wanted to throttle him. It twists you, manipulates you, and works you to its will, so even though it feels like your bad decisions, even though she can still welcome the guilt and feel the self-loathing of that, she knows that it was Twilight, because it's essentially this universal roofie.
So, at first, she's furious with him. He killed her girls. He ruined her life again, he killed her girls, he was evil, he was bad, he was killing people and hurting people and hurting her and he put her through hell all over again. But, then, she is brought to an equally horrible place, influenced by it, and for a while she's still riding the high of Twilight, but then it all comes down. Then, possessed by Twilight entirely now, he kills Giles. Then she destroys the root of all magic. Then, she gets it.
What he did was wrong. He screwed up, cosmically, and punishment is warranted. He opened the door to let this thing in because he was weak, because he was scared, because he had been beaten down beyond all belief in L.A. and he didn't know what else to do except let Twilight help him hope that there could be a light at the end of the tunnel. He let Twilight in. And he will hate himself for it, and Buffy knows it because she's seen it in him before, and she knows it because she knows he was in a horrible place to begin with to let Twilight in, and now it will be worse.
She forgives him for it. When she can't look at Angel in 8x40, it has nothing to do with Angel. Because she understands on a personal level exactly what Twilight's influence can do, and she doesn't hold that against him. She doesn't hate him. It's not a grudge. It's self-loathing.
To some degree, her forgiveness of Angel is hope that someone can forgive her for it, and on another level, it's trying to take all of the blame onto herself at the same time. Her inability to do anything but be a retarded cat with little 16-year-old girl feelings about Angel whenever he's around causes a deconstruction of her fairytale fantasy wherein they can regress to that together and hide from the world in each other. They do it (both the hiding and the more figurative translation of doing it) and they wind up literally hidden from the world, in another universe, a new universe.
And in doing so, they bring the Apocalypse. Buffy is singlehandedly (or, maybe not hand. Maybe single tonguedly. Or, vaginaedingly) responsible for the Apocalypse that was brought to a world she was destined to protect.
Even though she can forgive Angel and get past his actions, his killing of her girls, his involvement, explain them as being a part of Twilight's influence and thereby not his fault to justify forgiving him because God knows he needs it, she doesn't excuse herself, and that is why she can't stand to look at him.
She doesn't blame him, she doesn't hate him, but he's a reminder of what she did. Of her cosmic screw-up (and the puns just keep rolling). Buffy feels like season 8 was her ultimate failure as the Slayer. She messed up. She's done bad things before, attacked her friends and locked them in the basement, ruined a lot of girls' lives turning them into Slayers in the pursuit of saving the world. This was on a bigger scale, this was failing her duty. It's not the same as screwing some people over and having them be collateral damage in saving the world, or hurting her friends and letting them down. She failed.
And, it wasn't a failure because she wasn't strong enough, or a failure because she just couldn't do it. She failed because of a lack of self control, the way she perceives it (thought, given the daterape nature of Twilight, that's arguable). It wasn't the Slayer, it wasn't her power that wasn't enough, it was Buffy. And that's why she can't forgive herself, and that's why she seeks redemption now in a way that she never has needed to before.
That's why the last words of Angel: the Series are echoed in the last panel of season 8. She understands what it's like now, to not be able to forgive yourself and to need it more than anything in the world. She has her friends blaming her (chiefly Willow) and she blames herself and she is constantly surrounded by Slayers and wiccas who are all too eager to remind her of how colossally she screwed up. This is Buffy Summers' turn for feeling she needs to work for redemption and earn it, because she needs forgiveness and she doesn't have it in her to forgive herself ever.