This is a diagram of Snape's brain, in case you didn't know. I just found this today, and I automatically felt very sad. It's the kind of picture that's so accurate that you don't know whether to laugh or cry. And then if you laugh, you're going to cry anyway because you feel bad for laughing.
But this is just so true that it hurts. Snape's mindset would be like, Lily Lily Lily Lily Guilt Lily Lily I-hate-that-bastard-that-stole-Lily Lily Lily Lily Go-to-bathroom Lily Must-keep-her-son-safe Lily Lily Lily Potions Lily Mentally-scarring-students... And I'm not going on with that. But he has Occlumency to push sad memories away, so he doesn't have to feel that pain all the time.
God, I just feel so freaking sorry for him. My heart just broke a little more after seeing this.
