Shizuku talks real tough, but she's probably the biggest softie of the bunch. She's kinda trapped in the cycle of hating herself and finding reasons to justify that feeling. I think the others genuinely valuing her and encouraging her to value herself more are why Shizuku's more resistant to being dragged back in than she would have been in the past. But she's still weak to people who make her feel worthless, especially when they're able to play on that sense of worthlessness to create a sense of obligation.
Hope Saori is able to get through to her when she finds her.