Here’s the deal.
Ranger General Sylvanas ‘died’ when she was killed by Arthas. Part of her soul fractured, and that part of her soul was held by the Jailer within Torghast. What Arthas raised was a cold, mostly heartless being without any real humanity. That Banshee, while under the domination of Arthas/The Lich King, committed many atrocities and after she broke free of the Lich King’s domination she committed many more atrocities, all without any mental influence. She was doing these things by her own free will.
When she threw herself off of Icecrown after Arthas was defeated, she made a deal with the Jailer. That’s when she started working with him, as she saw their goals as being one and the same. She has continued to work with him as he acted in the Shadowlands, and burned Teldrassil to send a torrent of souls to him, which in turn gave her access to powers she never had before. Powers which allowed her to easily best Saurfang and Bolvar, powers that allowed her to destroy the Helm of Domination and open the way for the Jailer’s forces to invade Icecrown and capture various leaders.
It’s really important to note that she is doing this all because she wants to. She is not under the effects of the Jailer’s domination magic.
When the Jailer actually reveals his ‘master plan’ at the end of the Sanctum of Domination raid, Sylvanas realizes at this point that she’s been lied to. She and the Jailer never had the same goal, so she lashes out with that arrow, perhaps hoping that the Jailer would just kill her outright for her betrayal. Instead he forced the split part of her soul back into her body as a ‘gift’ before he entered the Zereth Mortis portal with Anduin.
That fragmented part of her soul is now subjected to reliving all the atrocities that Sylvanas committed while she was a banshee. Both while she was dominated by Arthas and while she was free and engaging in these actions herself. To Ranger General Sylvanas this is an unending torment, because she cannot accept that this Banshee is her. She refuses to accept it. And that division is what’s preventing Sylvanas from waking up.
This was intentionally done by the Jailer. Sylvanas knows of his goal and what he plans to do, and that makes her a liability now that she’s betrayed him. Clearly he hoped that Sylvanas would destroy herself and be unable to wake up to attempt to stop him. He, however, didn’t count on Uther aiding her, convincing her that the banshee and ranger general are one in the same, and eventually the ranger general agrees, accepting that they’re not two different people fighting for control, but rather, the same person and that she must face the consequences for her actions.
And that’s when she wakes up, because now the rift in her mind that was keeping her unconscious is gone.